The effect of different water discharge patterns from the artificial Oyan Reservoir in Ogun State, Nigeria, on water level fluctuations and on schistosomiasis transmission was investigated between August 1990 and March 1993 to determine the impact of water level regulation on schistosomiasis transmission and control. The results show that transmission was greatly influenced by the pattern of water discharge during the hot dry season (January-April). A high discharge during this period of no rainfall, high temperatures, and intense sunshine stimulated rapid water level fluctuations and lake draw-down, which led to significant reductions in all indices of schistosomiasis transmission, i.e. snail density, snail infection rates, human water contact patterns, and incidence of infection. Although these results support continuous water discharges from the reservoir during the hot dry season, this may run counter to current water management policies. Further investigation is therefore required to harmonize the potential benefits in this type of discharge pattern with the objectives of efficient water management in artificial reservoirs. Español