Variations in zooplankton biomass and population density in relation to the prevailing hydrographical conditions were studied in Zuari Estuary, Goa. The physico-chemical 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 decapods, brachyuran zoeae, cirripede larvae and chaetognaths. Lucifer hanseni formed an appreciable part of the total decapod population. Ten calanoid copepod species such as Metacalanus aurivilli, Pseudodiaptomus bowmani, Calanopia minor, Acartia bowmani, Monstrilla 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 diel and tidal cycles are discussed