The usefulness of a proposed biochemical key to the species of the Anopheles gambiae complex is tested against the standard chromosomal method in N.E. Tanzania. The chromosomal identifications indicate the presence of three sibling species of the complex in this area, partly in coexisting populations. The electromorph frequencies at the diagnostic enzyme loci, octanol-dehydrogenase (Odh) and superoxide dismutase (Sod), show neither significant geographical nor seasonal variation. It is confirmed that A. merus can be distinguished unambiguously on its Sod genotype. A. gambiae s.s. and A. arabiensis can be identifed, using the differences in their Odh polymorphism, with an average error of misidentification of 1 in 300. It is concluded that the biochemical key is sufficiently reliable to be a valuable additional taxonomic tool in the study area.