Nile tilapias; Oreochromis niloticus (L.) were selected from River Nile during autumn (2004) to estimate the impact of its stock density on some physicochemical characteristics and plankton assemblage inhabiting the Nile water. Results revealed slight decrease in pH values with increase of incubation time and fish densities. Ammonium-N was higher than nitrite and nitrate concentrations. Total organic nitrogen (TON) and total organic phosphorus (TOP) were much higher than the corresponding values of inorganic forms. Chlorophyceae, Bacillariophyceae and Cyanophyceae were the prevailing algae while Chrysophyceae, Cryptophyceae and Dinophyceae were scarcely occurred. The small zooplankon, like rotifers, especially Keratella cochlearis (Gosse)  and K. tropica (Apstein) were the most dominant organisms in the predation aquaria, Statistical analysis revealed the significant effect of fish densities and incubation time on green algae and diatoms. Also, the current data showed a significant impact of the stock fish density on zooplankton communities. Phytoplankton densities decreased gradually with time in aquaria stocked with 4 and/or 16 fishes and increased with that stocked with 8 or 12 fishes after 48 hours. Also, zooplankton communities increased after 48 hour. Therefore, the stock density of O. niloticus (L) should be increase from 0.8 to 1.6 g l^-1in fish farms derive its water from River Nile. Also, tilapia culture in fish farms can be depend to a large extent on natural plankton production from Nile water.