The rainfall over Lake Victoria is greatly influenced by the seasonal migration of the intertropical convergence zone (ITCZ), which is part of the monsoonal cycle over the Indian Ocean. Rainfall statistics during the rainy months April and November show that: (i) a maximum occurs on the western side of the lake in the early morning; and (ii) maxima occur on the eastern side in the late afternoon and around midnight. The factors determining these distributions are discussed. Introduction Lake Victoria is the biggest freshwater lake in the tropical regions of the world. It straddles the Equator and lies within the countries of Kenya, Tanzania and Uganda, having a surface area of approximately 70 000 km2. Compared to the other freshwater lakes of the world, it is the second largest, being next to Lake Superior. The River Nile originates from this lake at Jinja in Uganda and, passing through Lakes Kyoga and Mobutu Sese-Seko, is joined by the Blue Nile from Ethiopia and then flows through the Sudan into Egypt. The catchment area of the Upper Nile up to Nimule at the Uganda–Sudan border is 41 000 km2 of which 87%; lies in Kenya, Uganda and Tanzania and the remaining 13%; in Rwanda, Burundi and Zaire. Since the River Nile has a direct bearing on the economic development of all these countries, study of the water balance of Lake Victoria, which regulates the flow of the Nile, is of immense importance.