diff --git a/src/ProtocolBuffers.Test/ByteStringTest.cs b/src/ProtocolBuffers.Test/ByteStringTest.cs
new file mode 100644
index 0000000..768d74f
--- /dev/null
+++ b/src/ProtocolBuffers.Test/ByteStringTest.cs
@@ -0,0 +1,98 @@
+﻿// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.
+// http://code.google.com/p/protobuf/
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+using System.Text;
+using NUnit.Framework;
+
+namespace Google.ProtocolBuffers {
+  [TestFixture]
+  public class ByteStringTest {
+    [Test]
+    public void EmptyByteStringHasZeroSize() {
+      Assert.AreEqual(0, ByteString.Empty.Length);
+    }
+
+    [Test]
+    public void CopyFromStringWithExplicitEncoding() {
+      ByteString bs = ByteString.CopyFrom("AB", Encoding.Unicode);
+      Assert.AreEqual(4, bs.Length);
+      Assert.AreEqual(65, bs[0]);
+      Assert.AreEqual(0, bs[1]);
+      Assert.AreEqual(66, bs[2]);
+      Assert.AreEqual(0, bs[3]);
+    }
+
+    [Test]
+    public void IsEmptyWhenEmpty() {
+      Assert.IsTrue(ByteString.CopyFromUtf8("").IsEmpty);
+    }
+
+    [Test]
+    public void IsEmptyWhenNotEmpty() {
+      Assert.IsFalse(ByteString.CopyFromUtf8("X").IsEmpty);
+    }
+
+    [Test]
+    public void CopyFromByteArrayCopiesContents() {
+      byte[] data = new byte[1];
+      data[0] = 10;
+      ByteString bs = ByteString.CopyFrom(data);
+      Assert.AreEqual(10, bs[0]);
+      data[0] = 5;
+      Assert.AreEqual(10, bs[0]);
+    }
+
+    [Test]
+    public void ToByteArrayCopiesContents() {
+      ByteString bs = ByteString.CopyFromUtf8("Hello");
+      byte[] data = bs.ToByteArray();
+      Assert.AreEqual('H', data[0]);
+      Assert.AreEqual('H', bs[0]);
+      data[0] = 0;
+      Assert.AreEqual(0, data[0]);
+      Assert.AreEqual('H', bs[0]);
+    }
+
+    [Test]
+    public void CopyFromUtf8UsesUtf8() {
+      ByteString bs = ByteString.CopyFromUtf8("\u20ac");
+      Assert.AreEqual(3, bs.Length);
+      Assert.AreEqual(0xe2, bs[0]);
+      Assert.AreEqual(0x82, bs[1]);
+      Assert.AreEqual(0xac, bs[2]);
+    }
+
+    [Test]
+    public void CopyFromPortion() {
+      byte[] data = new byte[]{0, 1, 2, 3, 4, 5, 6};
+      ByteString bs = ByteString.CopyFrom(data, 2, 3);
+      Assert.AreEqual(3, bs.Length);
+      Assert.AreEqual(2, bs[0]);
+      Assert.AreEqual(3, bs[1]);
+    }
+
+    [Test]
+    public void ToStringUtf8() {
+      ByteString bs = ByteString.CopyFromUtf8("\u20ac");
+      Assert.AreEqual("\u20ac", bs.ToStringUtf8());
+    }
+
+    [Test]
+    public void ToStringWithExplicitEncoding() {
+      ByteString bs = ByteString.CopyFrom("\u20ac", Encoding.Unicode);
+      Assert.AreEqual("\u20ac", bs.ToString(Encoding.Unicode));
+    }
+  }
+}