AbstractThe water budget of Lake Naivasha, a fresh water lake at the bottom of Kenya's Rift Valley, just south of the equator at an altitude of c. 1885 m a.s.l. is discussed. It is suggested tentatively that the water budget of the lake to a large extent depends on groundwater flow to and from the lake. The latter point is very crucial, as the lake has no visible outlet. The water budget studies show that the potential evaporation is c. 2–3 times the amount of the annual precipitation over the lake. However, no studies so far have been made on the role thatSalvinia molestaMitch andCyperus papyrusL, which cover a great part of the lake, play in the water budget. This study indicates that the evaporation and transpiration from samples of the floating fernSalviniaapproximately equal the potential evaporation from a free water surface under similar conditions. Forpapyrusthe corresponding figure is about 200–250 %. The introduction ofSalviniain the 1960's has therefore hardly increased the water loss from the lake, whereas the influence ofpapyruson the water budget of the lake can not be disregarded. Nevertheless, the water budget studies and studies of the chemistry of the lake give strong support for the theory suggesting the existence of an underground outlet of the lake.