The seasonal fluctuations in the incidence of planktonic organisms and the larval forms of certain crustaceans such as crabs, barnacles and post larvae of prawns in the plankton of the south-west coast of India have been followed for a period of three years from 1963 . South-west monsoon period is the least productive period for zooplankton in this area . Brachyuran zoeae, post larvae of prawns and barnacle nauplii occur in the plankton throughout the year with distinct peaks for different groups . The zoeae of Uca annulipes occur in the plankton from September to May with a peak during November-December . The zoeae of Portunus pelagicus are present in the plankton from September to June and their abundance is in February-March . The post larvae of Metapenaeus affinis are found in the inshore plankton from November to June with the peak in March . The nauplii of Balanus amphitrite communis occur in the plankton in all the months of the year, the peak incidence has been during November January. The zoeae of U. annulipes are found to tolerate better the medium saline conditions . Of the ecological factors, salinity of the ambient water and the availability of planktonic food for the larvae seem to influence the seasonal fluctuations of these crustacean larvae in this locality.