Abstract: The present study deals with the study of the seasonal changes in phytoplankton population in Tungabhadra river from Harihar, Karnataka. From the selected 3 stations of Tungabhadra river water samples were collected at monthly intervals. Qualitative and quantitative analysis of phytoplankton were carried out during the year 2003-2004. About 71 species of phytoplankton were found comprising Chlorophyceae 35.47%, Bacilleriophyceae 18.11%, Cyanophyceae 37.48% and Euglenophyceae 8.93%. The peak of the phytoplankton population was observed during summer.