Every year, between March and August, the Onchocerciasis Control Programme (OCP) is invaded by large numbers of the onchocerciasis vector,Simulium damnosums.l., from breeding sites outside and to the south‐west of the programme area. One suspected source of this annual reinvasion lies in the mountainous area of South‐eastern Guinea along the headwaters of the Sassandra River. After a feasibility study, these watercourses were treated by helicopter with the organophosphate larvicide, temephos, from March to August 1985. It was shown that aerial treatments in this region of high mountains and forests were feasible and the operation produced over 90% reductions in biting and transmission in the reinvasion zones of Northern Ivory Coast and South‐western Burkina Faso. It was concluded that the Upper Sassandra tributaries in Southeastern Guinea were the principal source of vectors invading this region of the OCP and that these breeding sites must be treated annually to maintain biting and transmission in Ivory Coast and Burkina Faso at a tolerable level.