The methanogenesis of black liquor from pulp and paper mill was achieved using immobilized cell technology in a laboratory-scale two-stage reactor system run continuously for 340 days. The optimum organic loading rate for the anaerobic treatment of black liquor was 8.0kgm(-3)d(-1) at which the % COD removal, biogas production and methane content were 55%, 11dm(3)d(-1) and 71%, respectively. Organic loading rates above 8.0kgm(-3)d(-1) were observed to be toxic to the methanogenic bacteria and resulted in decreased methane content, biogas and COD removal. The applicability of the system to the large-scale processing and treatment of paper mill liquid waste is discussed.