Parthenium hysterophorus L. is an aggressive, invasive weed of some crops, wasteland and overgrazed pasture. Anaerobic fermentation of untreated and NaOH-treated Parthenium for CH4 production was investigated in 3 1 semi-continuous fermenters. The experiments included investigation of the effect of retention time, loading rate, feeding frequency and temperature on the steady-state methane production. Results showed that methane yield of 110 ml CH4 g-1 VS added and volumetric methane productivity of 456 ml CH4 l-1 day-1 were obtained for untreated Parthenium at 10 day retention time (HRT), 41.25 g VS l-1 and 30-degrees-C. The high liignin content of Parthenium was one of the factors accounting for the low methane yield. Alkali treatment of Parthenium with NaOH significantly enhanced the methane production and cellulose reduction. Results of pretreated fermenters indicated that, at low room temperature (24 +/- 2-degrees-C), feeding could be done on alternate days, which procedure established at HRT of 20 days and 41.25 g VS l-1 concentration. At 35-degrees and 40-degrees-C, the fermenters should be fed daily and operated at a HRT of 10 days and 41.25 g VS l-1. A maximum methane yield of 214 ml CH4 g-1 VS added and methane production of 883 ml CH4 l-1 day-1 were obtained at 10 day HRT and 40-degrees-C. The results of this study are projected to the family-size Deenbandhu biogas fermenter which has become popular recently in India and the energy production is estimated. The methane yield for Parthenium is compared with the reported yields of methane for weeds.