In aquatic ecosystems, planktons act as a bio-indicator and one of the crucial signal of environmental disturbances. Planktonic quantification from four sites of Pamaniyar river, Thiruvarur District, were assessed for seasonal variations. Phytoplankton as Cyanophyta, Chlorophyta, Bacillariophyta and Euglenophyta were found major groups. Comparative data related to abundance of species showed that, members of Bacillariophyta were dominated.