The paper reports on the biogas production from water hyacinth (Eichhornia crassipes) and channel grass (Vallisneria spiralis) employed separately for phytoremediation of lignin and metal-rich pulp and paper mill and highly acidic distillery effluents. These plants eventually grow well in diluted effluent up to 40% (i.e., 2.5-times dilution with deionized water) and often take up metals and toxic materials from wastewater for their metabolic use. Slurry of the two plants used for phytoremediation produced significantly more biogas than that produced by the plants grown in deionized water; the effect being more marked with plants used for phytoremediation of 20% pulp and paper mill effluent. Biogas production from channel grass was relatively greater and quicker (maximum in 6-9 days) than that from water hyacinth (in 9-12 days). Such variation in biogas production by the two macrophytes has been correlated with the changes in C, N and C/N ratio of their slurry brought by phytoremediation.