Seasonal and spatial distribution of phytoplankton in the pollution hit areas of the Periyer River tributary near the industrial belt of Alwaye have been studied. The station near FACT is noted for the total absence or poor concentration of phytoplankton; the high NH₃-N (20 to 500 µ g at l^-1) recorded at the station appears to inhibit the rate of photosynthesis. The remaining stations on either side show comparatively less NH₃-N concentrations and support more phytoplankton production.