diff --git a/requests/auth.py b/requests/auth.py
index d1196daa..3460c8b5 100644
--- a/requests/auth.py
+++ b/requests/auth.py
@@ -27,9 +27,15 @@ CONTENT_TYPE_MULTI_PART = 'multipart/form-data'
 
 def _basic_auth_str(username, password):
     """Returns a Basic Auth string."""
+    
+    if isinstance(username, str):
+        username = username.encode('latin1')
+
+    if isinstance(password, str):
+        password = password.encode('latin1')
 
     authstr = 'Basic ' + to_native_string(
-        b64encode(('%s:%s' % (username, password)).encode('latin1')).strip()
+        b64encode(b':'.join((username, password))).strip()
     )
 
     return authstr
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 1b063eed..a0c47eea 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -480,6 +480,20 @@ class TestRequests:
         r = s.get(url)
         assert r.status_code == 200
 
+    @pytest.mark.parametrize(
+        'username, password', (
+            ('user', 'pass'),
+            (u'имя'.encode('utf-8'), u'пароль'.encode('utf-8')),
+        ))
+    def test_set_basicauth(self, httpbin, username, password):
+        auth = (username, password)
+        url = httpbin('get')
+
+        r = requests.Request('GET', url, auth=auth)
+        p = r.prepare()
+
+        assert p.headers['Authorization'] == _basic_auth_str(username, password)
+
     @pytest.mark.parametrize(
         'url, exception', (
             # Connecting to an unknown domain should raise a ConnectionError
@@ -1573,10 +1587,15 @@ class TestRequests:
         self._patch_adapter_gzipped_redirect(s, url)
         s.get(url)
 
-    def test_basic_auth_str_is_always_native(self):
-        s = _basic_auth_str("test", "test")
+    @pytest.mark.parametrize(
+        'username, password, auth_str', (
+            ('test', 'test', 'Basic dGVzdDp0ZXN0'),
+            (u'имя'.encode('utf-8'), u'пароль'.encode('utf-8'), 'Basic 0LjQvNGPOtC/0LDRgNC+0LvRjA=='),
+        ))
+    def test_basic_auth_str_is_always_native(self, username, password, auth_str):
+        s = _basic_auth_str(username, password)
         assert isinstance(s, builtin_str)
-        assert s == "Basic dGVzdDp0ZXN0"
+        assert s == auth_str
 
     def test_requests_history_is_saved(self, httpbin):
         r = requests.get(httpbin('redirect/5'))