Total zooplankton biomass and the constituent species under the major taxa in relation to hydrographic conditions in the Hooghly estuarine complex were studied around Sagar Island during March 1979 to February 1981. Higher numerical counts and biomass of zooplankton occurred during high saline premonsoon period. During monsoon efflux most of the stenohaline organisms disappeard and only the low salinity species alongwith the euryhaline ones were able to thrive in the estuary. Copepods constituted the most predominant portion of zooplankton biomass throughout the year. Two peaks of zooplankton were invariably noticed-primary major peak during June/July and January/February showed the secondary major peak. © 1986 Indian Academy of Sciences.