Results of an investigation carried out on the spatio-temporal variation of zooplankton in the Bay of Bengal (Station 1) and Vellar Estuary (Station 2) from September 1997 to August 1998 are presented. Hydrological parameters such as water temperature, salinity, pH and dissolved oxygen were noticed in the ranges of 26.5-32.OC; 3-34%₀; 7.05-8.63 and 3.6-8.6 ml/l respectively. The inorganic nutrients like nitrate, nitrite, phosphate and silicate concentrations varied between 4.2 and 49.6 µM; 0.9 and 7.9 µM; 0.6 and 2.l µM and 7.9 and 139.9 µM respectively. Totally 50 species of zooplankton and 17 varieties of larval forms were identified from 2 stations. Of these, the copepods were the dominant group. The population density varied from 560 to 87,780 No./m³ at station 1 and 184 to 1,56,164 No./m3at station 2. Species diversity was higher (1.89-4.01 bits/individual) at station 2 than at station 1 (1.83-3.96 bits/individual). Higher values of plankton density and diversity were found during summer and post monsoon seasons and they were positively correlated with salinity and surface water temperature.