Synopsis The present comprehensive Study on Zooplankton diversity of the floodplain Lakes of Assam assumes special taxonomic and ecological importance in view of the stated lacunae. Plankton samples collected from 87 Beels of Assam are examined for analysis of the Faunal diversity of Zooplankton while investigations on their ecosystem diversity are based on regular seasonal collections from 30 Beels. Various documented taxa are briefly diagnosed and illustrated. Remarks are made on general Nature and composition of Zooplankton Fauna and their constituent groups, biogeographically interesting elements, indicator species, community similarities between different Beels and, Species diversity, dominance and evenness of dominant qualitative groups namely Rotifera and Cladocera. In addition, comments are made on influence of basic abiotic factors on richness and abundance of Zooplankton and their dominant groups.