An increase in the prevalence of schistomiasis in Egypt, as a result of the Aswan High Dam, has for long time been predicted because of reclamation of land and conversion of annual flooding (basin irrigation) into perennial irrigation in Upper Egypt. Two aspects, neglected in the predictions, have been investigated in the present study, and these are: the ecological changes, relevant to the transmission of the disease, in the Nile itself, and those in irrigation canals and drains in areas which had been under perennial irrigation before the constriction of the dam. The findings indicate that the Nile, at least in the Delta, has become a favorite habitat for breeding of the snail hosts of both urinary and intestinal schistosomiasis, a center for human activities and evidently for transmission, and that irrigation canals and drains now harbor throughout the year undisturbed and stable populations of these snail hosts. This has resulted from the elimination in these canals, of the so-called 'winter closure', because it is no more needed. Before the dam the closure was enforced for about 40 days, during which the canals were closed and dried up, and the silt deposited on their beds during the Nile flood dredged out together with the snails and aquatic weeds.