Phytoplankton abundance and composition were studied from the central and eastern Arabian Sea during three seasons namely inter-monsoon, winter and summer. Overall, phytoplankton population density was high during winter and summer periods and low during the inter-monsoon. Integrated cell numbers in the upper 120 m was from 0.4 to 1.2, 1.7 to 170 and 0.3 to 10 (X 10 sup(8)) cells m sup(-2) during inter-monsoon, winter and summer monsoon periods, respectively. Diatoms comprised 86% of the total population when three seasons were combined, followed by cyanobacteria (7%) and dinoflagellate (6%). The common diatoms were Nitzschia sp., N. seriata, N. longissima, Thalassiothrix spp., Rhizosolenia spp. and Chaetoceros spp. Some diatoms like Fragilaria sp., Thalassiothrix longissima, N. longissima, Rhizosolenia sp. and Chaetoceros pendulus were more abundant during the summer monsoon. The genera Fragilaria, Guinardia, Hemiaulus, Leptocylindrus and Lauderia appeared only during this season. A large number of diatoms were observed in sediment traps during winter.