Summary..In view of the growing importance of fish for native consumption, the resources of the African lakes are sure to be exploited in the near future, but it is essential to have a sound understanding of the fish and other fauna and flora in each lake before exploitation is started. This paper places on record the available data for Lake Rudolf and Lake Baringo, on which no previous ecological work had been attempted.Lake Rudolf is characterized by a high temperature and very alkaline water, and contains a variety of environments inhabited by different fish‐associations. The fauna has pronounced nilotic affinities, except that there is only one member of the family Mormyridae. Several species, however, have come to differ from their nilotic ancestors, and the comparison of the fauna with that of other waters can be used as evidence for past changes in the drainage systems of East Africa.The fish fauna of Lake Rudolf is roughly divisible into three main associations of species–one in the inshore waters, one in the open water, and one near the bottom. Prom data supplied by large numbers of stomach examinations, the food‐chains for each of these habitats can be reconstructed.Lake Baringo is less warm, the water has a relatively low alkalinity, and is uniformly shallow and muddy. The fish fauna consists of few species, and shows affinities with that of the East African rivers which flow to the Indian Ocean.Owing to the shallowness of Lake Baringo, there is no differentiation into species‐associations, and the food‐chains are complicated by the presence of food from the bottom in all the species of fish.The bulk of this paper contains analyses of field‐observations on the size, feeding and breeding habits of each of the species found i