Abstract: Zooplanktons are heterotrophic planktonic animals floating in water. They serve as good indicators of changes in water quality. The present study was undertaken to observe the seasonal fluctuations in diversity of zooplanktons of Achencovil river, a significant freshwater body in Kerala. Both qualitative and quantitative analysis of were conducted from July 2009 to June 2010, by taking samples from 3 segments of the river. Zooplankton community of Achencovil river comprised of 28 species belonging to Cladocera (11sps), Copepoda (9sps) and Rotifera (8sps). Cladocerans showed dominance both in number and diversity, followed by Copepods and Rotifers. This study also reveals that different groups of zooplanktons have their own peak periods of density, which is affected by local environmental conditions prevailing at that time.