Lake Naivasha is located northwest of Nairobi in the eastern Rift Valley (0 deg. 50' S, 36 deg. 20' E) at an altitude of 1900m. The catchment is volcanically active and the soils are derived from acidic and basic lava. The Malewa and Gilgil rivers are the major lotic systems in the basin, contributing more than 95% of the lake's inflow. Barnard & Biggs suggested that the macroinvertebrate fauna of these rivers was species poor for a tropical system and somewhat different (unique?) than systems surrounding the eastern Rift Valley. We investigated this hypothesis by quantitatively sampling the fauna at six riverine sites in the basin (Map) and comparing the fauna with that described in other East African rivers. Almost 90 taxa have currently been identified from these sites. Dipterans, particularly chironomids, were the most species-rich group, accounting for almost 40% of taxa; mayflies, caddisflies and beetles were next, each group accounting for about 15% of taxa. The number of taxa at sites with predominately sand substrate was similar to sites with predominately cobble substrate (i.e. 30-50 taxa), although the density was considerably less (300-1000 m-2 vs. 2000-5000 m-2 ). At cobble sites, dipterans and mayflies each accounted for 20-30% of the fauna, while beetles increased in relative abundance from 15 to 30% and caddis declined from 20 to 5% with declining elevation. At sandy sites, dipterans accounted for 50-60% of the fauna. None of the taxa were endemic to the basin or eastern Rift Valley. The fauna was similar to that found at similar elevations throughout East Africa, particularly the Ethiopian highlands.