As part of the return of savanna migrants installed since a long time in forest regions, in the south of Sierra Leone, we carried out an experimental study about a cross-transmission between Simulium sirbanum from Missira (West-Mali) and the forest strain of Onchocerca volvulus in the south-west of Sierra Leone. This study will allow to know if there is a risk of onchocerciasis transmission recrudescence in relation to the reinstallation of these migrants in their native region. Because of the very high limitation to the forest strain of O. volvulus microfilariae output of the peritrophic membrane reduction with savanna black-flies and according to the very low mature parasite out put of S. sirbanum with this strain observed along this experimentation, the forest strain of O. volvulus from the south Sierra Leone appears maladjusted to S. sirbanum, the main vector of onchocerciasis in savanna regions. This observation implicates a very low intensity of transmission for this forest strain by savanna onchocerciasis vectors. The return of savanna migrants in their native region, installed in the south Sierra Leone since several decades, could not be, in a short time, an origin of onchocerciasis recrudescence in savanna regions of the Onchocerciasis Control Programme area cleaned by an effective vector control carried out since 1975 sustained now by a chemotherapeutic treatment reducing the human parasite reservoir. However, the preservation of this acquired necessitates an epidemiological supervision increased, because the interactions between the vector and the parasite for a long time could carry away a mutual adaptation and a sickness recrudescence.