Nyumba ya Mungu reservoir is a man‐made lake in the Kilimanjaro region of northern Tanzania. The commercial fisheries are principally based on the cichlids,Sarotherodon panganiLowe,S. jipeLowe,S. esculentusGraham, andTilapia rendalliBoulenger.Examination of gut contents revealed that the dominant species,S. panganiandS. jipe, fed on the epiphyton attached to the vegetation in the littoral zone,S. esculentuswas a phytoplankton filter feeder, andT. rendalliconsumed macrophytes and associated epiphytes.A fish exclosure quantitatively demonstrated that the epiphyton community was appreciably depleted by the grazing of fish. After 35 days the mean epiphyton density on the submerged surfaces ofCyperus articulatusL. within the exclosure was 285 times 103individuals cm‐2shoot, but only 181 times 103individuals cm‐2shoot on grazed surfaces outside the exclosure. The differences were statistically significant, and corresponded to a difference in biomass of about 6.5 g organic dry weight m‐2lake surface. Vertical profiles of periphyton density onCyperusindicated that fish grazed mainly below a depth of 100 mm from the water surface.The littoral epiphyton → commercial fish food chain, which may be unique, is considered to predominate in Nyumba ya Mungu, and is compared with the major food chains of commercial fish in some other African natural and man