Abstract.In a 3‐year study during 1990–92, larval collections of theSimulium damnosumcomplex from the River Corubal system in Guinea Bissau revealed that the only sibling species present wereS. sirbanumand the Konkouré form ofS. konkourense, but notS. damnosum s.s.which had been found at some of the localities (Saltinho and Cusselinta) in a previous survey by Quillévéréet at.(1981). Their differential distributions were seasonally consistent between years.S. sirbanumwas concentrated in the upper reaches of the Corubal, moving downstream at the end of the dry season to exploit new breeding grounds, whereasS. konkourenseseemed to be concentrated in the lower reaches and moved upstream in the middle of the dry season.Since the previous survey, it appears thatS. konkourensehas largely replacedS. sirbanumandS. damnosum s.s.in the lower reaches of the Corubal.S. sirbanumwas consistently associated with man‐biting behaviour and, although infection studies were not performed, it is likely thatS. sirbanumis the main vector of onchocerciasis in Guinea Bissau asS. konkourensedoes not seen to be anthropophilic.Among five polymorphic inversions inS. sirbanum, the frequency of inversion IL‐B increased during the dry season; IL‐2 and IIL‐7 showed homozygous disadvantage whereas IIL‐3 and IIIL‐6 did not; IS‐2 tended to be fixed in all populations.Simulium konkourensepopulations in the River Corubal differ from those found upstream in its tributary the River Koumba, in the Fouta Djallon of Guinea, by their sex chromosomes (having no sex‐linked inversions) and biting preferences, indicating that the populations are not freely interbreeding. Inversion IIIL‐X was found for the first time in populations ofS. konkourenseoutside the Fouta Djallon area and a new inversion IIL‐Y was identified in samples from th