From july/1972 to november/1973 there were carried on the investigations on the problem of schistosomiasis mansoni in the Pampulha Lalce (Belo Horirizonte, MG., Brasil) especially regarding the rodents' epidemiological role in the area. In the 58 surveys carried out in the aforementioned period, 183 rodent specimens from 8 genera and 10 different species could be caught. Throught examination of their faeces and viscera (liver and intestine) it was found out that 10.9% (20) of those specimens harboured eggs and adult worms of Schistosoma mansoni. Such specimens belonged to just 3 species: Holochilus brasiliensis (Desmarest, 1818), Nectomys squarnipes squarnipes (Brants, 1827) and Zygodontomys lasiurus (Lund, 1841), each presenting, respectively, 11 (55.0%), 6 (30.0%) and 3 (15.0%) infected specimens. Nevertheless, in about 21 km along the lake perimeter, only 70 (0.4%) of the 16,090 ladles taken out presented planorbides, among which 64 specimens of Biomphalaria glabrata (Say, 1818) and 35 B. tenagophila (d'Orbigny, 1835) be found, all of them, showing to be negative for S. mansoni. The occurrence of parasitized rodents could be traced back to the cercariae shed by planorbides living in tributary brooks and trenches adjacents to them. Trematode larvae carried along by the water flow would reach the rodents in the "vital area" close to the lake. Through analogous process the lake usuaries might get infected despite the continuos vigilance by the local sanitary Authorities supervising the studies for a complete recovery of the area. The AA. assume that the rodents' role in the epidemiology of schistosomiasis will always vary from one area to another, in accordance with the specific features of each ecosystem involved.