Distribution and abundance of phytoplankton were studied from Adyar and Cooum estuaries

at Chennai for a period of one year (April 1998 to March 1999). Physical and chemical parameters like temperature, pH, dissolved oxygen, salinity and phosphate were analysed. The high phosphate (1.45 0.32 mg/l) content registered at Cooum estuary during postmonsoon season suggested that the freshwater influx could be the vital source of nutrients in the estuary. Among phytoplankton, diatoms were the dominant group represented by 43 taxa. The maximum abundance of total phytoplankton was recorded (12200 cells/l) during summer season at Cooum estuary. The low phytoplankton diversity and abundance are mainly due to the waste inflow through various freshwater input in the estuarine region.