Six diatoms achieve dominance in 26 alkaline, saline lakes in East Africa. There is a pronounced tendency for these species to replace each other as alkalinity increases. Cyclotella meneghiniana is usually dominant in less concentrated lakes and Nitzschia frustulum is favored as the alkalinity exceeds 80 meq liter−1. Coscinodiscus rudolfi and Navicula elkab are occasionally dominant at intermediate alkalinities. Navicula elkab is usually subdominant when N. frustulum is dominant. Most of these species when dominant are considered functionally planktonic in these waters. The presence of the cyanophyte Oscillatoria (Arthrospira) platensis in bloom seems to be a prerequisite for N. elkab and N. frustulum to enter the plankton. There is some evidence that the anionic composition of these lakes may be selective for some of the benthonic species.Geochemical data indicate that the production and preservation of diatom frustules appear to control silica concentrations in the waters. Relatively poor correlations were observed between sodium and silica and between pII and silica. These data have important implications for theoretical models of geochemical evolution in closed basins. Little or no dissolution of fossil diatom frustules was observed in the sediments of these highly alkaline, high pH waters.