Allele frequencies at enzyme loci were studied in Finnish populations of 2 spp. of the Anopheles maculipennis complex: A. beklemishevi and A. messeae. A. beklemishevi is spread over central and northern Finland, whereas A. messeae is found in southwestern and central Finland. The allele frequencies of these 2 spp. exhibit both similarities and differences. The 2 spp. do not interbreed in the nature. The allele frequencies at 2 loci, hydroxybutyrate dehydrogenase (Hbdh) and superoxide dismutase-2 (Su-2), are almost totally different and adult individuals of the 2 spp. can be reliably diagnosed by these allelic differences. The genetic distance, D, between A. beklemishevi and A. messeae is 0.35. This value is compared with corresponding distances between other dipterans studied.