An intensive monthly sampling of zooplankton carried out at a fixed station within the estuary of Potengi, Natal, Brazil, between April 1979 and March 1980 furnished information on the diurnal seasonal fluctuation of zooplankton components. It was observed that while tide and illumination brings about diurnal changes, seasonal rainfall and consequent dilution and enrichment of the ecosystem cause seasonal changes in the zooplankton population within the estuary. Copepoda, Appendicularia, Zoeae of Brachyura, nauplii of Cirripedia and Chaetognatha are major components in the samples. Among copepods, Acartia lillijeborgi, Pseudodiaptomus acutus, Paracalanus crassirostris and Calanopia Americana are dominant species.