Isozyme variation of the Simulium damnosum sibling species complex was studied by cellulose acetate electrophoresis (CAE) from four Kenyan river systems. Two enzymes, PGM and HK, were diagnostic and differentiated the larvae collected in Western and Nyanza provinces from the larvae collected at Mt. Kenya. Allele frequency differences of the enzyme PGI allowed about 75% separation of the geographically distinct populations.