The Hawkesbury-Nepean River is a regulated coastal river in New South Wales, Australia. Between March 1992and March 1993, the freshwater portion of the river was inhabited by a taxonomically rich (total: 116 taxa) anddense (annual mean community density: up to 1024 animals l−1) microzooplankton community, comparable tothat in some of the large regulated temperate rivers in the Northern Hemisphere. The common zooplankton taxa inthe river were similar to those observed in other rivers at the genus or species level, with a characteristic increasein protists (Vorticella spp.) towards the downstream reaches of the river. Zooplankton community density in theHawkesbury-NepeanRiver was, to some degree, predictable from river environmental variables: densitywas significantlynegatively correlated with river flow rate but positively correlated with temperature, turbidity, conductivity,total phosphorus and chlorophyll a. The results of the present study generally conform to the hypothesis of similarstructure of zooplankton communities in rivers, relative to that in lakes.