Abstract: An experiment on duckweed (Lemna minor) as supplementary feed in monoculture of nile tilapia (Oreochromis niloticus) was conducted for a period of 90 days. The experiment had two treatments each with three replications. In treatment-1 ponds were supplied with duckweed as supplementary fish feed and in treatment-2 ponds were kept as control (without supply of duckweed). Ponds were stocked at a stocking density of 80 fingerlings per decimal. The ponds were fertilized fortnightly with poultry dropping at the rate of 5 kg decimal-1, urea 60 g decimal-1 and TSP 90 g decimal-1. Duckweeds were supplied to the ponds (treatment-1) at the rate of 60% of the total body weight (wet weight basis) of the fish. During the experimental period, the ranges of physico-chemical parameters viz., water temperature, transparency, dissolved oxygen, pH, total alkalinity, free CO2, PO4-P and NO3-N were within the productive limit and more or less similar in all the ponds under treatments 1 and 2. There were 27 genera of phytoplankton under five major groups and 11 genera of zooplankton under three major groups were found in the experimental ponds. Mean survival rates in treatments 1 and 2 were 94.37 and 93.75%, respectively. Specific growth rate (SGR, % per day) of the fish in treatments 1 and 2 were 1.16 and 0.80%, respectively. Calculated net production of the fish in treatment-1 was 16.28 kg decimal-1 year-1 and in treatment-2 was 8.92 kg decimal-1 year-1. By t test it was found that the net production of fish in treatment-1 was significantly (p< 0.05) higher than that of treatment-2.