This article presents the planktonic biodiversity of the Dhepa River in Dinajpur district of Bangladesh. The research was conducted from September 2011 to April 2012 in six different sampling sties of the Dhepa River. A total of 31 species of phytoplankton were identified. Dominant phytoplankton group was Bacillariophyceae followed by Chlorophyceae, Cyanophyceae and Euglenophyceae. The phytoplankton biomass showed mainly two peaks in the present study, first peak in January and the second in April. Among 10 identified species of zooplankton Rotifer was the most dominant group. Like phytoplankton, zooplankton also showed two peaks, first in September and second in January. However, the overall plankton production reached to the highest level in the months of December to January. It might be due to low level of water depth and nutrient concentration of the river water.