The requirement of Ni2+ and CO2+ addition on methanogenic activity and the coenzymes involved in methanogenesis were investigated in anaerobic continuous cultivation with synthetic wastewater using acetate as the sole carbon source. Addition of Ni2+ and CO2+ to the synthetic wastewater drastically increased the maximum dilution rate of the cultivation. The concentrations of coenzymes F-430 and corrinoids in the biomass increased to 0.62 mu mol-Ni/g-VSS and 0.67 mu mol-Co/g-VSS, respectively with the increase of the dilution rate. Methanogenic activity of the culture broth also increased with an increase of dilution rate. However, without addition of Ni2+ and Co2+, F-430 and corrinoids were not detected in the biomass and methanogenic activity was only a trace level at a dilution rate of 0.025 d(-1). When the amounts of Ni2+ and Co2+ added at a dilution rate of 0.6 d-1 were lowered in steps, the concentrations of F-430 and corrinoids in the biomass and methanogenic activity decreased with decreasing amounts of Ni2+ and Co2+ added. These results suggest that Ni2+ and CO2+ were required for the methane-producing reactions via increases of coenzymes F-430 and corrinoids.