Lake Idku suffers from pollution from different sources; either agricultural or municipal wastes. It faces tremendous ecological stresses due to the environmental changes and threats originated from discharges of drains. So, the aim of the present study is to assess its pollution status according to the phytoplankton species distribution and water quality index. Eight water samples were collected from the lake, preserved and analyzed. The highest total count of phytoplankton was 1,865,000 unit L^-1 is at station 4 (at the southern part of the lake), while the lowest was in front of El-Boughaz. Phytoplankton species were belonging to four divisions namely; Euglenophyceae, Chlorophyceae, Bacillariophyceae and Cyanophyceae. The most abundant genera located at the highly polluted zones were Euglena spp. and Phacus spp.; at stations 4 and 7 (in front of El-Khairy drain); this is mainly due to drain wastes. Scenedesmus spp. (Chlorophytes), Cyctotella spp. and Nitzchia spp. (Bacillariophyceae),  Microcystis spp. and Chroococcus spp. were the most dominant at these stations, which are characterized by high levels of pollution. Members of euglenoids (Phacus spp. and Euglena spp.) were the most indicators to the pollution especially at stations 4 and 7 nearby drains, where they represented the main components of the total count of phytoplankton and characterized by high levels of ammonia and organic carbon. This was confirmed from the correlation matrix between phytoplankton with phosphate and ammonia. The national sanitation foundation water quality index (NSF-WQI) gives an indication to medium water quality status in lake Idku and refers to low value nearby El-Khairy drain that needs more treatment before discharging into the lake.