Electrophoresis of individuals of Anopheles funestus Giles, and a very closely related species from South Africa, and subsequent staining for non-specific esterases showed that the most intensely staining bands were limited to males. Dissection of individuals further showed these isozymes to be restricted to the accessory glands of the internal genitalia. The bands appear to represent the products of alleles segregating at a single. autosomal locus.