The epidemiology of human and bovine schistosomiasis in the Lake Sibaya area of Tongaland, South Africa, an undeveloped rural environment, is discussed. The mean prevalence of Schistosoma haematobium infection is 72%, but S. mansoni is absent; possible reasons for this are given and the different types of water habitat are shown to play different roles in transmission. Fear of crocodiles and hippopotami is important because villagers are compelled to use for domestic purposes, the smaller, shallower habitats, where Bulinus (Ph.) globosus occurs but not Biomphalaria pfeifferi, rather than the deeper, larger ones where both snails are found. This discontinuous distribution of B. pfeifferi in the deeper sites, due to the high temperatures in spring, is viewed in relation to local climatic conditions. A human population influx, such as would result from the economic development of Tongaland, would cause not only a decline in crocodile and hippopotamus populations, and thus encourage human contact with the deeper waterbodies, but would favour the introduction of S. mansoni into at least some of these habitats. A similar prevalence pattern of human schistosomiasis exists over the remainder of Tongaland and the lowlands of southern and central Mozambique. The mean prevalence of Schistosoma mattheei in Tongaland cattle is 42% and although data are not available a comparable percentage seems likely in Mozambique.