diff --git a/src/waitress/utilities.py b/src/waitress/utilities.py
index 6ae47420..a9d33618 100644
--- a/src/waitress/utilities.py
+++ b/src/waitress/utilities.py
@@ -262,8 +262,8 @@ def to_response(self):
         status = "%s %s" % (self.code, self.reason)
         body = "%s\r\n\r\n%s" % (self.reason, self.body)
         tag = "\r\n\r\n(generated by waitress)"
-        body = body + tag
-        headers = [("Content-Type", "text/plain")]
+        body = (body + tag).encode("utf-8")
+        headers = [("Content-Type", "text/plain; charset=utf-8")]
 
         return status, headers, body
 
diff --git a/tests/test_functional.py b/tests/test_functional.py
index 212e247b..2ac8d11e 100644
--- a/tests/test_functional.py
+++ b/tests/test_functional.py
@@ -359,7 +359,7 @@ def test_broken_chunked_encoding(self):
                 sorted(headers.keys()),
                 ["connection", "content-length", "content-type", "date", "server"],
             )
-            self.assertEqual(headers["content-type"], "text/plain")
+            self.assertEqual(headers["content-type"], "text/plain; charset=utf-8")
             # connection has been closed
             self.send_check_error(to_send)
             self.assertRaises(ConnectionClosed, read_http, fp)
@@ -381,7 +381,7 @@ def test_broken_chunked_encoding_invalid_hex(self):
                 sorted(headers.keys()),
                 ["connection", "content-length", "content-type", "date", "server"],
             )
-            self.assertEqual(headers["content-type"], "text/plain")
+            self.assertEqual(headers["content-type"], "text/plain; charset=utf-8")
             # connection has been closed
             self.send_check_error(to_send)
             self.assertRaises(ConnectionClosed, read_http, fp)
@@ -403,7 +403,7 @@ def test_broken_chunked_encoding_invalid_extension(self):
                 sorted(headers.keys()),
                 ["connection", "content-length", "content-type", "date", "server"],
             )
-            self.assertEqual(headers["content-type"], "text/plain")
+            self.assertEqual(headers["content-type"], "text/plain; charset=utf-8")
             # connection has been closed
             self.send_check_error(to_send)
             self.assertRaises(ConnectionClosed, read_http, fp)
@@ -428,7 +428,7 @@ def test_broken_chunked_encoding_missing_chunk_end(self):
                 sorted(headers.keys()),
                 ["connection", "content-length", "content-type", "date", "server"],
             )
-            self.assertEqual(headers["content-type"], "text/plain")
+            self.assertEqual(headers["content-type"], "text/plain; charset=utf-8")
             # connection has been closed
             self.send_check_error(to_send)
             self.assertRaises(ConnectionClosed, read_http, fp)
@@ -1121,7 +1121,7 @@ def test_request_body_too_large_chunked_encoding(self):
             self.assertline(line, "413", "Request Entity Too Large", "HTTP/1.1")
             cl = int(headers["content-length"])
             self.assertEqual(cl, len(response_body))
-            self.assertEqual(headers["content-type"], "text/plain")
+            self.assertEqual(headers["content-type"], "text/plain; charset=utf-8")
             # connection has been closed
             self.send_check_error(to_send)
             self.assertRaises(ConnectionClosed, read_http, fp)
diff --git a/tests/test_task.py b/tests/test_task.py
index cc579b03..47868e15 100644
--- a/tests/test_task.py
+++ b/tests/test_task.py
@@ -869,7 +869,7 @@ def test_execute_http_10(self):
         self.assertEqual(lines[0], b"HTTP/1.0 432 Too Ugly")
         self.assertEqual(lines[1], b"Connection: close")
         self.assertEqual(lines[2], b"Content-Length: 43")
-        self.assertEqual(lines[3], b"Content-Type: text/plain")
+        self.assertEqual(lines[3], b"Content-Type: text/plain; charset=utf-8")
         self.assertTrue(lines[4])
         self.assertEqual(lines[5], b"Server: waitress")
         self.assertEqual(lines[6], b"Too Ugly")
@@ -885,7 +885,7 @@ def test_execute_http_11(self):
         self.assertEqual(lines[0], b"HTTP/1.1 432 Too Ugly")
         self.assertEqual(lines[1], b"Connection: close")
         self.assertEqual(lines[2], b"Content-Length: 43")
-        self.assertEqual(lines[3], b"Content-Type: text/plain")
+        self.assertEqual(lines[3], b"Content-Type: text/plain; charset=utf-8")
         self.assertTrue(lines[4])
         self.assertEqual(lines[5], b"Server: waitress")
         self.assertEqual(lines[6], b"Too Ugly")
@@ -902,7 +902,7 @@ def test_execute_http_11_close(self):
         self.assertEqual(lines[0], b"HTTP/1.1 432 Too Ugly")
         self.assertEqual(lines[1], b"Connection: close")
         self.assertEqual(lines[2], b"Content-Length: 43")
-        self.assertEqual(lines[3], b"Content-Type: text/plain")
+        self.assertEqual(lines[3], b"Content-Type: text/plain; charset=utf-8")
         self.assertTrue(lines[4])
         self.assertEqual(lines[5], b"Server: waitress")
         self.assertEqual(lines[6], b"Too Ugly")
@@ -919,7 +919,7 @@ def test_execute_http_11_keep_forces_close(self):
         self.assertEqual(lines[0], b"HTTP/1.1 432 Too Ugly")
         self.assertEqual(lines[1], b"Connection: close")
         self.assertEqual(lines[2], b"Content-Length: 43")
-        self.assertEqual(lines[3], b"Content-Type: text/plain")
+        self.assertEqual(lines[3], b"Content-Type: text/plain; charset=utf-8")
         self.assertTrue(lines[4])
         self.assertEqual(lines[5], b"Server: waitress")
         self.assertEqual(lines[6], b"Too Ugly")