In present study, potentials of water hyacinth (Eichhornia crassipes) and water chestnut (Trapa bispinnosa) employed for phytoremediation of toxic metal rich brass and electroplating industry effluent, were examined in terms of biogas generation. Inability of the plants to grow in undiluted effluent directed to select 20%, 40% and 60% effluent concentrations (with deionized water) for phytoremediation experiments. Slurry of both the plants used for phytoremediation produced significantly more biogas than that by the control plants grown in unpolluted water; the effect being more pronounced with plants used for phytoremediation of 20% effluent. Maximum cumulative production of biogas (2430 c.c./100 g d m of water hyacinth and 1940 c.c./100 g d m of water chest nut) and per cent methane content (63.82% for water hyacinth and 57.04% for water chestnut) was observed at 5 min particle size and 1: 1 substrate/inoculum ratio, after twenty days incubation. Biogas production was quicker (maximum from 8-12 days) in water hyacinth than in water chestnut (maximum from 12-16 days). The qualitative and quantitative variations in biogas production were correlated with COD, C, N, C/N ratio and toxic metal contents of the slurry used. (c) 2006 Elsevier Ltd. All rights reserved.