The newly formed man-made Wadi El-Raiyan lakes are the immense reservoir for agriculture drainage water of El-Faiyoum Province. The present investigation was conducted to through some light on phytoplankton and their distribution in the lakes and their seasonal variation, population density and the species composition. The present data revealed that the phytoplankton inhabiting the lakes are belonging to the following divisions Chlorophyceae, Cyanophyceae, Bacillariophyceae, Dinophyceae and Euglenophyceae. The Chlorophyta was dominant class in all seasons except in the Autumn when the Cyanophyta predominated. A total of 96 taxa of phytoplankton belonging to 53 genera and 26 families and five classes were recorded during this investigation. The result revealed a positive correlation between phytoplankton standing crop and chlorophyll a values. The peak of both chlorophyll a and phytoplankton density was observed in Autumn, while the minimum value for both of them was recorded in Spring.