--- a/babel/dates.py
+++ b/babel/dates.py
@@ -468,6 +468,8 @@
             return self.format(self.value.minute, num)
         elif char == 's':
             return self.format(self.value.second, num)
+        elif char == 'S':
+            return self.format_frac_seconds(self.value.microsecond, num)
         elif char in ('z', 'Z', 'v'):
             return self.format_timezone(char, num)
         else:
@@ -518,6 +520,10 @@
         period = {0: 'am', 1: 'pm'}[int(self.value.hour > 12)]
         return get_period_names(locale=self.locale)[period]
 
+    def format_frac_seconds(self, char, num):
+        value = str(self.value.microsecond)
+        return self.format(round(float('.%s' % value), num) * 10**num, num)
+
     def format_timezone(self, char, num):
         if char in ('z', 'v'):
             if hasattr(self.value.tzinfo, 'zone'):