diff --git a/objectivec/Tests/GPBUnknownFieldSetTest.m b/objectivec/Tests/GPBUnknownFieldSetTest.m
new file mode 100644
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+++ b/objectivec/Tests/GPBUnknownFieldSetTest.m
@@ -0,0 +1,259 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#import "GPBTestUtilities.h"
+
+#import "GPBUnknownField_PackagePrivate.h"
+#import "GPBUnknownFieldSet_PackagePrivate.h"
+#import "google/protobuf/Unittest.pbobjc.h"
+
+@interface GPBUnknownFieldSet (GPBUnknownFieldSetTest)
+- (void)getTags:(int32_t*)tags;
+@end
+
+@interface UnknownFieldSetTest : GPBTestCase {
+ @private
+  TestAllTypes* allFields_;
+  NSData* allFieldsData_;
+
+  // An empty message that has been parsed from allFieldsData.  So, it has
+  // unknown fields of every type.
+  TestEmptyMessage* emptyMessage_;
+  GPBUnknownFieldSet* unknownFields_;
+}
+
+@end
+
+@implementation UnknownFieldSetTest
+
+- (void)setUp {
+  allFields_ = [self allSetRepeatedCount:kGPBDefaultRepeatCount];
+  allFieldsData_ = [allFields_ data];
+  emptyMessage_ = [TestEmptyMessage parseFromData:allFieldsData_ error:NULL];
+  unknownFields_ = emptyMessage_.unknownFields;
+}
+
+- (GPBUnknownField *)getField:(int32_t)number {
+  return [unknownFields_ getField:number];
+}
+
+// Constructs a protocol buffer which contains fields with all the same
+// numbers as allFieldsData except that each field is some other wire
+// type.
+- (NSData*)getBizarroData {
+  GPBUnknownFieldSet* bizarroFields =
+      [[[GPBUnknownFieldSet alloc] init] autorelease];
+  NSUInteger count = [unknownFields_ countOfFields];
+  int32_t tags[count];
+  [unknownFields_ getTags:tags];
+  for (NSUInteger i = 0; i < count; ++i) {
+    int32_t tag = tags[i];
+    GPBUnknownField* field = [unknownFields_ getField:tag];
+    if (field.varintList.count == 0) {
+      // Original field is not a varint, so use a varint.
+      GPBUnknownField* varintField =
+          [[[GPBUnknownField alloc] initWithNumber:tag] autorelease];
+      [varintField addVarint:1];
+      [bizarroFields addField:varintField];
+    } else {
+      // Original field *is* a varint, so use something else.
+      GPBUnknownField* fixed32Field =
+          [[[GPBUnknownField alloc] initWithNumber:tag] autorelease];
+      [fixed32Field addFixed32:1];
+      [bizarroFields addField:fixed32Field];
+    }
+  }
+
+  return [bizarroFields data];
+}
+
+- (void)testSerialize {
+  // Check that serializing the UnknownFieldSet produces the original data
+  // again.
+  NSData* data = [emptyMessage_ data];
+  XCTAssertEqualObjects(allFieldsData_, data);
+}
+
+- (void)testCopyFrom {
+  TestEmptyMessage* message = [TestEmptyMessage message];
+  [message mergeFrom:emptyMessage_];
+
+  XCTAssertEqualObjects(emptyMessage_.data, message.data);
+}
+
+- (void)testMergeFrom {
+  GPBUnknownFieldSet* set1 = [[[GPBUnknownFieldSet alloc] init] autorelease];
+  GPBUnknownField* field = [[[GPBUnknownField alloc] initWithNumber:2] autorelease];
+  [field addVarint:2];
+  [set1 addField:field];
+  field = [[[GPBUnknownField alloc] initWithNumber:3] autorelease];
+  [field addVarint:4];
+  [set1 addField:field];
+
+  GPBUnknownFieldSet* set2 = [[[GPBUnknownFieldSet alloc] init] autorelease];
+  field = [[[GPBUnknownField alloc] initWithNumber:1] autorelease];
+  [field addVarint:1];
+  [set2 addField:field];
+  field = [[[GPBUnknownField alloc] initWithNumber:3] autorelease];
+  [field addVarint:3];
+  [set2 addField:field];
+
+  GPBUnknownFieldSet* set3 = [[[GPBUnknownFieldSet alloc] init] autorelease];
+  field = [[[GPBUnknownField alloc] initWithNumber:1] autorelease];
+  [field addVarint:1];
+  [set3 addField:field];
+  field = [[[GPBUnknownField alloc] initWithNumber:3] autorelease];
+  [field addVarint:4];
+  [set3 addField:field];
+
+  GPBUnknownFieldSet* set4 = [[[GPBUnknownFieldSet alloc] init] autorelease];
+  field = [[[GPBUnknownField alloc] initWithNumber:2] autorelease];
+  [field addVarint:2];
+  [set4 addField:field];
+  field = [[[GPBUnknownField alloc] initWithNumber:3] autorelease];
+  [field addVarint:3];
+  [set4 addField:field];
+
+  TestEmptyMessage* source1 = [TestEmptyMessage message];
+  [source1 setUnknownFields:set1];
+  TestEmptyMessage* source2 = [TestEmptyMessage message];
+  [source2 setUnknownFields:set2];
+  TestEmptyMessage* source3 = [TestEmptyMessage message];
+  [source3 setUnknownFields:set3];
+  TestEmptyMessage* source4 = [TestEmptyMessage message];
+  [source4 setUnknownFields:set4];
+
+  TestEmptyMessage* destination1 = [TestEmptyMessage message];
+  [destination1 mergeFrom:source1];
+  [destination1 mergeFrom:source2];
+
+  TestEmptyMessage* destination2 = [TestEmptyMessage message];
+  [destination2 mergeFrom:source3];
+  [destination2 mergeFrom:source4];
+
+  XCTAssertEqualObjects(destination1.data, destination2.data);
+}
+
+- (void)testClearMessage {
+  TestEmptyMessage *message = [TestEmptyMessage message];
+  [message mergeFrom:emptyMessage_];
+  [message clear];
+  XCTAssertEqual(message.serializedSize, (size_t)0);
+}
+
+- (void)testParseKnownAndUnknown {
+  // Test mixing known and unknown fields when parsing.
+  GPBUnknownFieldSet *fields = [[unknownFields_ copy] autorelease];
+  GPBUnknownField *field =
+    [[[GPBUnknownField alloc] initWithNumber:123456] autorelease];
+  [field addVarint:654321];
+  [fields addField:field];
+
+  NSData* data = fields.data;
+  TestAllTypes* destination = [TestAllTypes parseFromData:data error:NULL];
+
+  [self assertAllFieldsSet:destination repeatedCount:kGPBDefaultRepeatCount];
+  XCTAssertEqual(destination.unknownFields.countOfFields, (NSUInteger)1);
+
+  GPBUnknownField* field2 = [destination.unknownFields getField:123456];
+  XCTAssertEqual(field2.varintList.count, (NSUInteger)1);
+  XCTAssertEqual(654321ULL, [field2.varintList valueAtIndex:0]);
+}
+
+- (void)testWrongTypeTreatedAsUnknown {
+  // Test that fields of the wrong wire type are treated like unknown fields
+  // when parsing.
+
+  NSData* bizarroData = [self getBizarroData];
+  TestAllTypes* allTypesMessage =
+      [TestAllTypes parseFromData:bizarroData error:NULL];
+  TestEmptyMessage* emptyMessage =
+      [TestEmptyMessage parseFromData:bizarroData error:NULL];
+
+  // All fields should have been interpreted as unknown, so the debug strings
+  // should be the same.
+  XCTAssertEqualObjects(emptyMessage.data, allTypesMessage.data);
+}
+
+- (void)testUnknownExtensions {
+  // Make sure fields are properly parsed to the UnknownFieldSet even when
+  // they are declared as extension numbers.
+
+  TestEmptyMessageWithExtensions* message =
+      [TestEmptyMessageWithExtensions parseFromData:allFieldsData_ error:NULL];
+
+  XCTAssertEqual(unknownFields_.countOfFields,
+                 message.unknownFields.countOfFields);
+  XCTAssertEqualObjects(allFieldsData_, message.data);
+}
+
+- (void)testWrongExtensionTypeTreatedAsUnknown {
+  // Test that fields of the wrong wire type are treated like unknown fields
+  // when parsing extensions.
+
+  NSData* bizarroData = [self getBizarroData];
+  TestAllExtensions* allExtensionsMessage =
+      [TestAllExtensions parseFromData:bizarroData error:NULL];
+  TestEmptyMessage* emptyMessage =
+      [TestEmptyMessage parseFromData:bizarroData error:NULL];
+
+  // All fields should have been interpreted as unknown, so the debug strings
+  // should be the same.
+  XCTAssertEqualObjects(emptyMessage.data, allExtensionsMessage.data);
+}
+
+- (void)testLargeVarint {
+  GPBUnknownFieldSet* fields = [[unknownFields_ copy] autorelease];
+  GPBUnknownField* field = [[[GPBUnknownField alloc] initWithNumber:1] autorelease];
+  [field addVarint:0x7FFFFFFFFFFFFFFFL];
+  [fields addField:field];
+
+  NSData* data = [fields data];
+
+  GPBUnknownFieldSet* parsed = [[[GPBUnknownFieldSet alloc] init] autorelease];
+  [parsed mergeFromData:data];
+  GPBUnknownField* field2 = [parsed getField:1];
+  XCTAssertEqual(field2.varintList.count, (NSUInteger)1);
+  XCTAssertEqual(0x7FFFFFFFFFFFFFFFULL, [field2.varintList valueAtIndex:0]);
+}
+
+- (void)testMergingFields {
+  GPBUnknownField* field1 = [[[GPBUnknownField alloc] initWithNumber:1] autorelease];
+  [field1 addVarint:1];
+  [field1 addFixed32:2];
+  [field1 addFixed64:3];
+  [field1 addLengthDelimited:[NSData dataWithBytes:"hello" length:5]];
+  [field1 addGroup:[[unknownFields_ copy] autorelease]];
+  GPBUnknownField* field2 = [[[GPBUnknownField alloc] initWithNumber:2] autorelease];
+  [field2 mergeFromField:field1];
+  XCTAssertEqualObjects(field1, field2);
+}
+
+@end