Abstract. Abstract Aim This study was carried out to determine which phytoplankton species, as a natural food, can be ingested and digested by Nile tilapia (Oreochromis niloticus L.). Methods During this study, phytoplankton in the gut contents of Nile tilapia collected from three fishponds in southern Egypt were investigated during the period Oct. 2012-Sep. 2013. Samples of tilapia fish were grown in aquarium containing filtered pond water to detect undigested phytoplankton species in the feces. Results The majority of the phytoplankton found in the gut of Nile tilapia was Cyanobacteria (36-50%) and Chlorophyta (27-38%). Other groups such Diatoms, Euglenophyta and Dinophyta were also found but with lower percentages (<19%). The most important and dominant phytoplankton species found in Tilapia gut were the potentially toxic cyanobacteria, Anabaena, Anabaenopsis, Cylindrospermopsis, Microcystis and Planktothrix. Only diatoms were recorded in the feces, indicating the ability of Tilapia to digest all phytoplankton except diatoms. Conclusions The data of this study could be useful for biomanipulation of nuisance phytoplankton blooms in eutrophic aquacultures.