The effects of different stocking densities of freshwater prawn (Macrobrachium rosenbergii) on its growth and production in relation to the presence of small self-recruiting species mola (Amblypharyngodon mola) were investigated in modified rice fields after rice harvest at Mymensingh, Bangladesh. Keeping the stocking density of mola fixed at 20 000 ha−1 in each treatment, four densities of freshwater prawn (treatments) were maintained: 10 000, 15 000, 20 000 and 25 000 ha−1 respectively. The rice plots were limed (CaCO3) and fertilized with urea, triple super phosphate and cowdung regularly. The prawns were fed daily with commercial pellets. Water quality parameters such as temperature, dissolved oxygen, transparency, pH, total alkalinity, inorganic nitrogen (nitrate, nitrite and total ammonia), chlorophyll a and orthophosphate were determined fortnightly. Numerical analysis of plankton communities was performed monthly. All water quality parameters were found to be within the suitable range for freshwater prawn culture, except high temperature, in the peak summer months. There was a homogenous abundance of plankton communities in all treatments throughout the experimental period. Mola started to breed in the second month of the original stocking with partial harvesting after the second month and continued until the final harvest. The total production of mola ranged between 124 and 152 kg ha−1 during the 4-month culture period. The average prawn survival ranged from 49% to 57% without any significant difference among treatments. Freshwater prawn production ranged from 294 to 596 kg ha−1 with significantly higher production in the treatment where 20 000 ha−1 freshwater prawn were stocked. This treatment also resulted in a higher net profit margin (74%), indicating that stocking at a combination of 20 000 ha−1 freshwater prawn and 20 000 ha−1 mola could be the optimum proposition for prawn–mola culture in modified rain-fed rice fields after rice cultivation.