We surveyed Yellow Rails (Coturnicops noveboracensis) in three marshes located in southeastern James Bay, Canada, in order to document the status and distribution of this species at risk. Night-surveys of calling males were made along 75 km of line transect on 21-25 July 2002, and 186 individual Yellow Rails were recorded: 80 in Cabbage Willows Bay, 77 in Boatswain Bay, and 29 in Hall Cove. Daytime surveys yielded 19 additional calling males, giving a total of 205 male rails. Transects were distributed in plant communities dominated by Slimstem Reedgrass (Calamagrostis stricta), Chaffy Sedge (Carex paleacea), Buckbean (Menyanthes trifoliata), or by Red Fescue (Festuca rubra). There was an overall density of 0.05 calling rails per ha with maximum densities in Boatswain Bay (0.08 male/ha) and Hall Cove (0.06 males/ha). Densities calculated for specific plant communities permitted an estimate of 397 male Yellow Rail inhabiting the marshes: 216 in Boatswain Bay, 132 in Cabbage Willows Bay, and 49 in Hall Cove. The highest densities were in Chaffy Sedge stands, Slimstem Reedgrass stands, and fens dominated by Buckbean. This study indicates that the Yellow Rail is a common bird in coastal high-marshes of the southeastern James Bay, and revealed the highest densities ever reported for the species. The extensive estuarine tidal marshes found in southern James Bay may be home of a thousand or more adult male Yellow Rails and thus represents a key area for this species.