  We present the results of a search for optical identifications of
ultraluminous X-ray sources (ULXs) in M51 by using mosaic images taken with the
Hubble Space Telescope Advanced Camera for Surveys in filters F435W (B), F555W
(V), F814W (I), and F658N (Halpha). Our sample consisting of nine ULXs is
defined by analyzing the three Chandra observations of M51 performed in 2000
June, 2001 June, and 2003 Aug. We found four ULXs have one or two candidates
for counterparts, while two have multiple stars within their error circles. The
other three have no candidate counterparts. Four ULXs are located near or in a
star cluster, while others have no association with a cluster. These results
indicate that the companion star, environment, and origin of ULXs are probably
heterogeneous.
