The siphonal index and the parameters used in its determination were compared for late fourth instar larvae of Culex pipiens subspecies reference strains, field-collected intermediates, and laboratory hybrids. Groups of larvae from egg rafts of Culex pipiens pipiens (CPP) and Culex pipiens quinquefasciatus (CPQ) could be distinguished from each other by measurement of length or width or their siphons as well as by the ratio of length to width (the siphonal index). Groups of larvae from egg rafts of intermediates similar to Cx. p. pipiens (CPP*) differed primarily in siphon width from those similar to Cx. p. quinquefasciatus (CPQ*); intermediates differed primarily in siphon length from the subspecies each resembled. Laboratory hybrids (F1) obtained from crossing the two subspecies were indistinguishable from the CPP* intermediate under the rearing conditions used. Siphon width was positively correlated with siphon length, and the slopes of the regressions for these correlations were similar for all subsets of the Culex pipiens complex.