In 1998, the African Programme for Onchocerciasis Control (APOC) decided to launch a long-term impact assessment of its operations. This paper reports the baseline entomological data collected throughout a whole year in two sites of Cameroon (Kahn and Bolo). The Simulium populations of the two study sites were characterized by parous rates of 7.2% and 33.5% respectively and infectivity by O. volvulus of 31 and 190 infective larvae per 1000 parous flies respectively The Annual Transmission Potentials (ATP) were respectively 523 and 9972 infective larvae per man and per year in Kahn and in Bolo. The Simulium populations studied in both sites, even though the ATP in Kahn is 19 times lower than that of Bolo, showed a pattern of an onchocerciasis hyperendemic zone in terms of vector capacities and entomological indices.