Index: Lib/test/test_StringIO_jy.py
===================================================================
--- Lib/test/test_StringIO_jy.py	(revision 5912)
+++ Lib/test/test_StringIO_jy.py	(working copy)
@@ -12,9 +12,17 @@
         self.assertEqual(u'foo', cStringIO.StringIO(u'foo').read())
         self.assertEqual('foo', cStringIO.StringIO(u'foo').read())
 
+class TestWrite(unittest.TestCase):
+    def test_write_seek_write(self):
+        f = cStringIO.StringIO()
+        f.write('hello')
+        f.seek(2)
+        f.write('hi')
+        self.assertEquals(f.getvalue(), 'hehio')
 
 def test_main():
     test_support.run_unittest(TestUnicodeInput)
+    test_support.run_unittest(TestWrite)
 
 if __name__ == '__main__':
     test_main()
Index: src/org/python/modules/cStringIO.java
===================================================================
--- src/org/python/modules/cStringIO.java	(revision 5912)
+++ src/org/python/modules/cStringIO.java	(working copy)
@@ -325,9 +325,40 @@
 
         public synchronized void write(String s) {
             _complain_ifclosed();
-            buf.setLength(pos);
-            int newpos = pos + s.length();
-            buf.replace(pos, newpos, s);
+
+            int spos = pos;
+            int slen = buf.length();
+
+            if (spos == slen) {
+                buf.append(s);
+                buf.setLength(slen + s.length());
+                pos = spos + s.length();
+
+                return;
+            }
+
+            if (spos > slen) {
+                int l = spos - slen;
+                char[] bytes = new char[l];
+
+                for (int i = 0; i < l - 1; i++)
+                    bytes[i] = '\0';
+
+                buf.append(bytes);
+                slen = spos;
+            }
+
+            int newpos = spos + s.length();
+
+            if (newpos > slen) {
+                buf.replace(spos, slen - spos, s);
+                buf.append(s.substring(slen));
+                slen = newpos;
+            } else {
+                buf.replace(spos, spos + s.length(), s);
+            }
+
+            buf.setLength(slen);
             pos = newpos;
         }