The Culex pipiens complex consists of several species, subspecies, forms, races, physiological variants, or biotypes according to different authors and includes the 2 holarctic variants Cx. pipiens biotype pipiens and Cx. pipiens biotype molestus. Differences in morphological characters are overlapping and thus are delimited in their taxonomic value, even when behavioral and reproductive specializations are apparent. Our enzyme electrophoretic study included 7 geographic populations of Cx. pipiens biotype pipiens and 7 of the biotype molestus from several European countries. For comparison, 5 populations of Culex quinquefasciatus from Asia, Africa, and North America were examined. The aim was an assessment of the extent of genetic differences between local populations of the biotypes pipiens and molestus versus the degree of differentiation between geographic populations of both groups. Culex torrentium, Cx. modestus, Culex stigmatosoma, and Culex territans were studied for comparison as taxonomical well-defined species. The population genetic analyses revealed much higher genetic distances between local populations of Cx. pipiens biotype pipiens and Cx. pipiens biotype molestus compared to the low differentiation between geographic populations within each taxon. The UPGMA analysis and F-statistics position the geographic populations in discrete monophyletic clusters. Gene flow between local populations of the biotypes pipiens and molestus could be shown to be lower than gene flow between geographically distant populations within each of the 2 groups, leading to the conclusion that Cx. pipiens biotype molestus could be a distinct taxon. Culex quinquefasciatus could be diagnosed as genetically well separated, in particular by the diagnostic enzyme marker MDH (NADP). Two genetic enzyme markers were identified to differentiate Cx. torrentium from Cx. pipiens s.l. Culex modestus, Cx. stigmatosoma, and Cx. territans showed considerable genetic distances to the species of the Culex pipiens complex and between each other, and several genetic markers could be identified.