The helminth fauna of Oreochromis leucosticus (Trewavas) from Lake Naivasha is described. The study was carried outon 1,050 gillnetted fish caught over a period of 24 months at different sites within the lake. Oreochromis leucosticuswhich is a commercially important fish was observed to be a host of four helminth parasites. These were anacanthocephalan polyacanthorhynchus kenyensis (Schmidt and Canaris), a nematode contracaecum sp.. a trematodeclinostomum sp. and a dilepidid cestode Amirthalingamia sp.Seasonal variation in prevalence, intensity of infection, organs infected and the variation in infection with the size, sex and body condition of the host are described. There was no seasonal variation in prevalence but intensity of infection was observed to increase with the age of the fish. Although male fish were observed to be more heavily infected, the parasites did not seem to have any significant effect on the body condition of the fish