The present work focused on physico-chemical parameters, diversity and seasonal abundance of phytoplankton in Tungabadhra reservoir Bellary District for the period of February 2009 to January 2010. We have observed monthly fluctuations of different diversity i.e 21algal genera belong to five groups, of which 4 genera of cynophycease, 5 genera of chlorophyceae, 4 genera of euglenophyceae, 1 genera of desmid and 6 genera of diatoms. Among the plankton diatoms were the dominant group throughout the study period. The high numerical abundance of phytoplankton individuals was recorded in southwest monsoon season and northeast monsoon season while low density was observed in the summer season. The seasonal fluctuations, correlation with physico-chemical variable, diversity indices and evenness indices were analyzed for all plankton.