1. The general features and topography of Lake McIlwaine in Rhodesia are described, including a description of the main aquatic vegetational communities and benthic deposits. 2. The thermal regime is described and compared with observations on similar water bodies in the South-Central African region. It is concluded that these artificial lakes are all of the warm monomictic type, but verge on the polymictic owing to the cooling effect of summer storms. 3. The waters show fairly high levels of nitrite- and nitrate-nitrogen as a result of drainage of sewage effluents into the lake. It is believed that the abundance of plankton in the lake is a direct result of the mild pollution. There is no evidence that pollution has at this stage reached a serious level, although the profundal waters suffer severe anaerobiosis during most of the year. 4. Fauna lists of the zooplankton, benthic and littoral invertebrates and fishes are given. 5. It has been demonstrated that the sampling errors resulting from the action of the Van Veen grab can be successfully countered by statistical methods, giving improved estimates of the abundance of the randomly distributed oligochaete, pelecypod and chironomid populations. 6. The mean standing crop of benthic invertebrates is estimated conservatively at between 200 and 230 kg/hectare. Total annual benthic production is estimated at just over 1000 metric tons, or 400–450 kg/hectare. The estimates are based only upon sampling in the 2–5 m zone of the upper sub-littoral and although carefully considered are nevertheless presented with some reservations. If the estimates do err, they are probably conservative. 7. The maximum standing crop of benthic invertebrates is found in the 2–5 m zone in the upper sub-littoral. The profundal region is almost completely unproductive.