The variations of phytoplankton with reference to abundance and distribution have been studied in relation to temperature, salinity and nutrients in the Nethravathi estuary. Trimodal distribution pattern of phytoplankton was observed with higher production during May, June July and Nov.- Dec. The phytoplankton biomass varied between 1132 and 65514 cells/m3. Diatoms were the major components of phytoplankton followed by green algae, dinoflagellates, blue green algae) flagellates. Green algae were more abundant during the periods of low temperature and salinity while diatoms, dinoflagellates and blue green algae were relatively higher during higher conditions and salinity.