The azo-dye method for the histochemical demonstration of acid phosphatase activity was used to differentiate filarial larvae within and outside th area of the Onchocerciasis Control Programme (OCP) in natural infections of female S. damnosum s.l. caught in West Africa. The histochemical patterns of 1263 larvae (all stages) dissected from 556 positive files caught at 35 catching sites during the period of reinvasion in 1978 and 1979 were determined and compared with those of O. volvulus known from experimental infections. In Mali, Ivory Coast and Upper Volta, about 16% of 3rd-stage larvae in 17.3% of invading female S. damnosum s.l. (savanna cytospecies) could be separated from those of O. volvulus-like larvae, on account of their different enzyme staining patterns. The percentage of larvae enzymatically distinguishable from O. volvulus and the flies carrying them showed a distinct geographical distribution; the highest percentages (36.4/38.4) were found in the north-west (Mali) and the lowest percentages (4.4/8.2%) were found in the interior (east-central) of the Programme area (Upper Volta). By contrast, all larvae found in S. damnosum s.l. females caught in Ghana and in Togo were morphologically as well as enzymatically similar to those of O. volvulus. Third-stage larvae of the enzymatically distinguishable "species" were found to be somewhat longer than those of O. volvulus-like larvae. Morphologically, the larvae concerned probably belong to the genus Onchocerca, but their specific identity and vertebrate host remain unknown.