It is now widely recognized that development of irrigation projects in endemic areas for schistosomiasis invariably leads to a recrudescence of the disease by increasing the habitat of the intermediate snail host of Schistosoma mansoni. This fatality was again demonstrated by experience in the Katana region of the Democratic Republic of Congo where development of 43 fresh water reservoirs for raising Tilapia nilotica led to multiplication of Biomphalaria pfeifferi. A parasitological study was conducted in three new villages around these basins and in neighboring villages. Stool examinations were performed in a total of 787 people. Infestation rates were 8.1 p. 100 and 4 p. 100 respectively. Infestation exceeded 25 p. 100 in children between the ages of 10 and 14 years. These findings underline the need for preventive measures.