When 2,4,6-TCP (trichlorophenol) as a toxicant was added to the reactors with 500, 1000, 2000 and 4000 mg COD/l, methane production ratios between the reactors with and without toxicant were 64, 75, 83 and 96%. The 2,4,6-TCP was more toxic to methane production at COD concentrations lower than 1000 mg/l. In continuous operation, when the toxicant was fed to the reactor, methane production rate (CH4-l/g-COD) recovered in four days.