Culex (Culex) pipiens L. and Culex (Cx.) torrentium Martini are sympatric sibling species ordinarily inseparable at egg, larval or pupal stages. Adult males of both species may however be distinguished on the basis of their differential terminalia, and the females on the basis of the presence or absence of pre-alar patch of scales on the sternopleuron. The latter character is believed to be lacking in Cx. pipiens. In this study newly emerged adults of both species, obtained from field egg batches, have been closely examined and compared. The presence of pre-alar scales in Cx. pipiens adults is reported, and the unreliability of this character as a diagnostic feature clearly indicated. It is therefore suggested that in distinguishing adult females of Cx. pipiens and Cx. torrentium eggs should be obtained and the progeny reared to give F1 males which could be conclusively identified by the structure of the terminalia.