The productivity of an aquatic environment is directly correlated with the density of plankton. The plankton population in any aquatic system is biological wealth of water for fishes and constitutes a vital link in the food chain. Bisalpur reservoir has an ecosystem contain biodiversity of planktons and fishes. The present study was undertaken to investigate the plankton diversity in Thadoli area of Bisalpur reservoir having distance approximate 20-25 km., through different months during the period of February 2008 to January 2009. During the present study the most pollution tolerant species of Oscillatoria, Euglena and Navicula were recorded. Among the zooplanktons, Rotifers are good indicators of water quality. Rotifers of genus Brachionus and Keratella are abundant in water of the reservoir. Their occurance in eutrophic water is well documented . The species composition and species diversity study also points towards the polluted nature of reservoir water. There is no earlier study on the diversity of plankton of this lentic water resource and that is why present study was planned.