The Collaborative International Dictionary of English v.0.48:

co-worker \co`-work"er\, coworker \co`work"er\(k?`w?rk"?r), n.
   One who works with another; a cooperator.

   Syn: colleague, coworker, fellow worker, fellow-worker,
        workfellow.
        [1913 Webster + WordNet 1.5]