The occurrence and abundance of diatoms in the nearshore waters of the bay in relation to physicochemical characteristics of waters was studied. Two stations were chosen for collecting samples. Station 1 was located midway between Adyar River and Cooum River and station 2 opposite the mouth of river Cooum. Weekly surface samples were made and hydrological date collected. Phytoplankton that occurred i» the collections were analysed for diatoms and 34 diatoms were identified and their abundance was correlated with the existing hydrological conditions. Dissolved oxygen content was low at high temperature and high salinity. pH was dependent on changes in total CO₂ and this in turn was dependent on diatom population. At station 2 low salinity generally coincided with dense diatom population while at station 1 there was no such correleation. The standing crop of phytoplankton was high. Availability of high amount of nutrients could well be the cause for high production. Studies on growth characteristic of some diatoms and blue green algae in the laboratory revealed that while diatoms seemed to thrive better under existing hydrological conditions blue green algae preferred a medium very rich in nitrogen nutrients