Investigations on intestinal schistosomiasis were carried out in Yoro, a small village located in the transitional zone between forest and savannah, in the Mbam and Inoubou Division of Cameroon. Four human-water contact points were identified in the village and sampled for snails, and the inhabitants underwent parasitologic and clinical surveys to search for signs and symptoms of intestinal schistosomiasis. The results indicated the presence of two freshwater snails, both potential intermediate hosts of Schistosoma sp: Biomphalaria pfeifferi and Bulinus forskalii. However, only the former species was incriminated in the transmission of the disease, with the prevalence of snail infection being 10% (1 of 10) and 14.3% (2 of 14), respectively, during surveys 1 (in the dry season) and 2 (in the rainy season). The overall prevalence of Schistosoma mansoni eggs in stool samples was 54.4% (98 of 180). The mean +/- SD intensity of infection was 100.3 +/- 114.7 eggs per gram of stool. Eggs of S. intercalatum were not detected during parasitologic examination of stool specimens. In Cameroon, it appears that unlike the distribution of S. mansoni, which usually follows that of B. pfeifferi, B. forskalii is commonly found where S. intercalatum does not exist due to competitive exclusion through introgressive hybridization. Of the 180 people included in the study, 52.3% reported abdominal pain and 37.5% had bloody stools. Splenomegaly and hepatomegaly were noted in 11.7% and 3.9%, respectively, of the subjects examined. Three foci of S. mansoni were previously described in the Mbam and Inoubou Division, including Bafia town, Makenene, and Kinding Djabi villages. With the present focus in Yoro, the Mbam and Inoubou Division appears to be the most important endemic zone of S. mansoni in southern Cameroon.