The present study investigated gut contents of the chela fish (Salmophasia bacaila) and environmental variability at the Dhepa River in Bangladesh. Monthly sampling of fish and water quality parameters were performed at three different sites in the Dhepa River from September 2017 to February 2018. PERMANOVA indicated a significant temporal variation (PERMANOVA, P=0.001) in water temperature, dissolved oxygen and pH. Water transparency showed spatio-temporal variation among the months and sites (PERMANOVA, P=0.001) in the Dhepa River during the study period. A total of 20 genera of phytoplankton and seven genera of zooplankton were recorded from the gut of chela fish. Phytoplankton was highly dominant (92.9%) prey item, while zooplankton contributed only 7.1% to the diets of chela. PERMANOVA showed a significant temporal (PERMANOVA, P=0.01) variation in the prey diversity of chela fish. Overall, the result indicates that chela fish is predominantly plankton feeder. Changes in prey diversity can be influenced by the variation in abundance and distribution of preys that are likely regulated by the environmental factors in the Dhepa River. This study improves our understanding on prey selection and diversity of a small indigenous fish from a riverine system.