Productivity in the coastal waters of Cochin was found to be inversely proportional to the size of planktonic algae except in the case of the size groups of < 53 - 25,um and < 25 - 10 um. This may be due to the selective grazing of these size fractions by zooplankton, which in turn reduce the standing crop of planktonic algae and hence lower the rate of their primary production. Generally, during the pre and the post-monsoon seasons, the major contribution was that of the planktonic algae < 25 p in size. This may be due to the relatively low concentration of nutrients in the coastal waters. Smaller microalgae, due to their higher efficiency to absorb nutrients, grow well and reproduce better than the larger microalgae. During the monsoon season, the microalgae >10 pm in size contribute significantly to the total primary production, as there is high concentration of nutrients in the coastal waters, which is essential for the growth and reproduction of the larger microalgae.