Variations in zooplankton biomass and population density in relation to the prevailing hydrographidal conditions were studied in Zuari estuary, Goa. The physicochemical parameters showed limited variations. Zooplankton biomass was relatively more in samples collected during night while it was higher in day collections. Copepods formed the predominant group (30.7 - 69.6%) followed by decapod., brachyuran zoeae, cimpede larvae and chaetognaths. Lucifer hameni formed an appreciable part of the total decapod population. Ten calanoid copepod species such as Metacalanus aurivilli, Pseudodiaptomus bowmani, Calanopia minor, Acartia bowmani, Monshilla sp. obtained in day collections and Pseudodiaptomus sewelli, P. Jonesi, Calanopia elliptica, C. herdmani and Acartia pacifica in the night collections are the first record from the Zuari estuary. Variations in the incidence of the common zooplankton groups and their species over the die1 and tidal cycles are discussed.

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