SUMMARYBetween August 1978 and July 1979, freshwater gastropods were collected at monthly intervals from 9 different non-lacustrine habitats in the Mwanza region of Tanzania. Of a total of 11 708 gastropods representing 14 species, 1748 (14'9 %) were infected with trematode sporocysts and/or rediae. Altogether 38 morphologically distinguishable' species' of cercariae were recovered (13 furcocercous, 10 xiphidiocercaria, 6 echinostome, 4 cystophorous, 3 gymnocephalous and 2 amphistome species), 22 of which did not conform to previously described African species. The majority (63'8 %) of all mature infections were xiphidiocercariae. Biomphalaria pfeifferi, B. sudanica and Ceratophallus natalensis each yielded 11 species of cercariae. Lymnaea natalensis had the highest overall prevalence of infection (36'9 %). Cercaria guttera from L. natalensis accounted for 20·4 % of all recovered trematode infections and C. blukwa from Biomphalaria accounted for 18'4 % of all infections; the high prevalence of these two xiphidiocercariae may alter the transmission patterns of Fasciola gigantica and Schistosoma mansoni, respectively. S. mansoni was recovered from both B. sudanica (22 of 2393 infected) and B. pfeifferi (79 of 1913 infected); S. haematobium (or related species) was recovered from Bulinus (Physopsis) nasutus (50 of 1503 infected) and to a lesser extent from B. (P.) africanus (6 of 186 infected). The findings are discussed in relation to the biological control of trematode diseases in Tanzania.