ABSTRACT.The Beffa form ofSimulium soubrenseVajime&Dunbar, a member of theS.sanctipaulisub‐complex of theS.damnosumcomplex, was found breeding throughout rivers in the Togo‐Benin Gap, as far north as 9o30'N. Its distribution changed with the season. In southern Togo there were seasonal fluctuations in the relative abundancies of the Beffa form and ofS.damnosumlS.sirbanum.There was considerable temporal and regional variation in the frequencies of different colour morphs of adult flies. The flies in Benin tended to be darker. Infections withOnchocerca volvulus(Leuckart) appeared to be independent of the host's colour morph category. Larger flies harboured significantly more first stageOnchocercalarvae but no significant relations with fly size were found for second and third stage lar