Ennore estuary was investigated from 1976-1978 with special reference to hydrobiology and zooplankton contents. The estuary is characterised by very few channels of fresh water inflow and prolonged period of saltwater domination. Among the zooplankton, copepods, decapod larvae, cladocerans, medusae and fish eggs constitute the main component. Majority of zooplankton shows maximum occurrence and abundance during the high salinity period. The fish landing were mainly represented by mullets and prawns. Several interesting findings like the presence of Evadne sp. and Penilia sp., occurrence of chaetognaths, medusae and fish eggs in abundance during a particular season have been noticed down during the investigation.