Siphonal indices of Culex pipiens complex field strains were compared. Collection locales included those where only Culex pipiens pipiens or Cx. p. quinquefasciatus occur and a study area (Memphis, Tennessee) within the zone where intermediates occur between these subspecies. It proved possible to identify single egg rafts as Cx. p. pipiens, Cx. p. quinquefasciatus or one of two types of intermediates. The method of precisely measuring siphonal index is described and uses of the method are discussed along with questions that need to be answered about the Culex pipiens complex.