An on-farm experiment was carried out during 2005–07 to study the impact of fish and prawn rearing on rice field ecology and productivity in rice–fish–prawn system. Water pH, total alkalinity, total suspended solid, plankton and chlorophyll-a were significantly lower in the rice mono crop than the rice–fish–prawn system, where supplemental feed was provided. However increased phytoplankton and chlorophyll-a concentration in the rice–fish–prawn system did not help in maintaining higher dissolved oxygen levels compared to rice monocrop probably due to decreased autotrophic/ increased heterotrophic activity. Gut content analysis of the cultured species indicated plenty availability of planktonic, periphytic and benthic food to fish and prawn in the rice field (22.7–48.9%), that reduce the supplemental feed input. Rice grain yield of 3.04 tonnes/ha in the rice–fish–prawn system was 16.9% higher than the rice monocrop. In rice– fish–prawn system, when 50% area is devoted for fish and prawn culture, the net returns enhanced by 23-folds in comparison to rice monocrop. Significantly higher net returns of Rs 79 585/ha, net water productivity of Rs 7.66/m3 and the higher ratio of the output value to the cost of cultivation (1.6) in the rice–fish–prawn system infers that rice– fish–prawn culture being more beneficial can be adopted and expanded in lowland/ waterlogged areas.