A research conducted for 1 year, the feeding selectivity of Oreochromis niloticus was investigated in 7 sampling sites in the Lakes Naivasha and Oloiden. The fish samples of O. niloticus were analyzed in the laboratory. The water samples collected for phytoplankton and zooplankton were also analyzed under a compound microscope. Nile tilapia's feeding selectivity of plankton was further analyzed using Ivlev index.