The phytoplankton diversity of Harda swamp of Purnia (India) in relation to certain physicochemical factors was studied. A total of 27 genera of phytoplankton, belonging to four groups – chlorophyceae, myxophyceae, bacillariophycae and euglenophyceae were found in the swamp. The quantitative relationship amongst the groups of phytoplankton is chlorophyceae >myxophyceae > bacillariophycae > euglenophyceae. The phytoplankton density in different seasons was in order of summer > winter > monsoon. The species diversity of phytoplankton was found maximum in monsoon (H= 1.297) followed by winter (H= 1.289) and summer (H= 1.222). The species belonging to chlorophyceae were dominant. Pollution tolerant species found in swamps are stagnation of waters for the most of the period and limited water recharge. The swamp harbours many allergenic algae such as Chlorella vulgaris, Anabaena sp., Microcystis sp., Nostoc sp. and Oscillatoria sp. The swamps harbour many pollution tolerant species like Cyclotella, Fragilaria and Navicula. Phytoplanktons showed negative correlation with pH, DO and bicarbonate. On the basis of Palmer’s algal index the swamp water is polluted.
