Oreochromis niloticlls L. is commercially the most important fish in Ethiopia. Despite itsrnimportance, little information is available on the biology and ecology of this fish in mostrnwater bodies in Ethiopia. This thesis is a contribution to understanding the feedingrnecology and importance of diet on the lipid quality of fish with emphasis on O. niioticlIsrnin Lakes Chamo and Langeno. The natural food of O. niloticlIs were studied, based onrnmonthly samples collected, during December 1995 and December 1996 from both lakes.rnThe diet of Chamo fish was mainly composed of phytoplankton. Cyanobacteria,rnboth filamentous (e.g. Anabaena, Anabaenopsis, Raphidiopsis, Oscillatoria) and coccoidrnforms (e.g. ChrococclIs and Microcystis), were found to be the most important foodrnitems. Diatoms, mainly Navicula, and Nitzschia and green algae (e.g. Cosmarium andrnScenedesmus) also constituted the diet. Macrophytes were found in abundance during thernrainy season while food of animal origin was only encountered occasionally. Significantrnseasonal variations (ANaYA p