Electrophoretically-detected variation at 18 enzyme protein structural loci in members of the Culex pipiens group of species is summarized using Rogers' genetic distance algorithm and presented as a similarity dendrogram. The two native Australian species C. australicus Dobr. and Drum. and C. globocoxitus Dobr. are more similar to each other than either is to C. quinquefasciatus Say, C. molestus Forsk., C. pallens Coq. and European C. pipiens L. Within this latter group of species C. quinquefasciatus separates from the other three. The two basic clusters represent those species with rickettsial endosymbiotes, and those without these organisms. It is argued that the symbiotic-free condition is the ancestral one, and that the two clusters of species represent separate evolutionary lines that diverged during the Cretaceous.