Parthenium hysterophorus L. is a terrestrial weed on culturable wasteland and cropland. It grows luxuriantly in many parts of India creating agricultural and health hazards. The effect of inoculum size and pretreatments on methane (CH4) production from Parthenium was investigated in 21 batch fermenters at room temperature (26 +/- 2 degrees C). Fresh Parthenium (FP) produced a maximum yield of 152 +/- 15 ml CH4 g(-1) volatile solids (VS) added at the highest volume of inoculum (1000 ml) tested. At inoculum/substrate (I/S) ratios (on a volume/VS basis, ml I/g VS S) below 67, the CH4 yields from FP for 35 days were significantly (P < 0.05) lower than the maximum value. Dried Parthenium (DP) at 49.5 g VS l(-1) and 1000 ml inoculum, to give an I/S ratio of 67, required 21 days to produce 140 +/- 8 ml CH4 g(-1) VS added at 26 +/- 2 degrees C. Though the inoculum used with DP differed significantly (P < 0.05) in solids content from the inoculum used with FP, an I/S ratio close to 67 seemed to be sufficient for assessing the biodegradability of FP and DP. Thus short-term batch fermentation of Parthenium at 26 +/- 2 degrees C might be possible with high volume of inoculum to give an I/S ratio of 67. Of the several pretreatment techniques, there was no significant (P < 0.05) effect of heat treatment on the CH4 yield. The CH4 yields from the HCl and NaOH treated Parthenium were 45 and 69%, respectively, higher than the untreated Parthenium.