The biological effects of impounding water by a dam have been investigated on the White Nile, by taking a series of plankton samples from the Gebel Aulyia Dam upstream for 352 km. From the sampling station farthest from the dam to the dam itself, the plankton showed a numerical increase of more than 100-fold, the phytoplankton rising from 49 to 6790 x 103 and the zooplankton from 0.8 to 123 organisms per litre. This increase has been related to the changes from river to lake-like conditions. The maximum density of both phyto- and zooplankton was attained in the last 50 km above the dam. In the upper regions the plankton was mixed with non-planktonic forms, while of the truly plankton forms, diatoms and copepods predominated. With increasingly lake-like conditions the adventitious forms, disappeared and Cyanophyceae and Cladocera gained dominance. Although there was a loss of suspended detritus, turbidity increased owing to the vast development of phytoplankton near the dam. The dissolved oxygen concentration and the pH also showed marked increases, which have also been correlated with the rich phytoplankton near the dam.