This study was carried out in Gowrikere tank of Anandapura Shivamogga city, Karnataka on phytoplankton diversity and density for a period of twelve months from January to December 2012. A total of 48 species belonging to 37 genera of phytoplankton were recorded. Relative abundance of phy of Bacillariophyceae (27.08%), followed by Chlorophyceae (22.91%), Cyanophyceae (20.83%), Desminds (18.75%) and Euglenophyceae (10.41%). The highest density of phytoplankton was recorded during summer season. Chlorococcale (10925 org. /l (Organism/Litre)) during May and lower in the month of December (7034 org. /l). Diatoms ranged from 1981 org. /l (September) to 4432 org. /l (January). Desmids va peak density (83 org./l) during February and lower in the month of July (32 org. /l). Euglenoids population showed maximum density during the month of March (3091 org with 1973 org./l in the month of July. Density of blue greens recorded a maximum of 9114 organism / in April and a minimum of 7115 org tolerant species such as Scenedesmus quadricauda, Coelastrum microporum, Phacus longicauda, Trachelomonas robasta and Microcystis aeroginosa were recorded. INTRODUCTIONP h yt o p l a nk t o n s a r e p h ot o s yn t h e s i z i ng microscopic organisms that inhabit the upper sunlit layer of almost all oceans and bodies of freshwater. They form bulk of food for zooplankton, fishes and other aquatic organisms. Phytoplankton the primary level of the aquatic food chain by converting solar radiant energy into biomass through photosynthesis. Therefore, they act as primary produces and represent themselves as a direct food source for the aquatic animals. They also play an important role in conditioning the microclimate by regulating the level of dissolved oxygen, a vital gas for aquatic life (Sudhakara 2012