A chromosomal identification of the Stimulium damnosum complex larvae collected from areas within and outside the Onchocerciasis Control Programme (OCP) area revealed six species of the complex. Five of these (S. sirbanum, S. damnosum s.s., S. soubrense, S. yahense, S. squamosum) were observed within and south of the OCP area. S. sanctipauli was found exclusively outside the OCP zone. S. sirbanum: Predominated in the Sudan Savanna, spread into the Guinea and transition zones but was almost absent in the forest region. S. damnosum s.s.: Abundant in the Guinea Savanna; was well represented in the Sudan Savanna while smaller populations penetrated the forest region. S. sanctipauli: Identified almost exclusively from the forest zone; a few savanna foci were found on the upper course of the Sassandra R. in the Ivory Coast and on the Volta R. within the coastal savanna strip in Ghana. S. soubrense: Recorded from the forest, transition and Guinea Savanna zones; northern limits were established at the Léraba Bridge on the Upper Volta/Ivory Coast border. S. yahense: Predominantely distributed along creeks and small rivers in the forest region; a few records within the Guinea Savanna were found on the upper reaches of the Sassandra valley. S. squamosum: Observed in small rivers in the forest zone; extensively colonised heavily shaded or forested areas within the Guinea Savanna but in the Sudan Savanna only two records were observed near Bobo-Dioulasso in Upper Volta.