Laboratory experiments were conducted to investigate the performance of the anaerobic sequencing batch reactor (ASBR) for digestion of a municipal sludge. The reactors were operated at an HRT of 10 days with an equivalent loading rate of 0.8-1.5 g VS 1−1 d−1 at 35°C. Solids were accumulated rapidly in the ASBR during start-up period. Flotation thickening occurred in the ASBRs, and its efficiency was comparable to that of additional thickening of the completely mixed control reactor. Solids concentrations in the ASBRs were 2.6 times higher than that in the control. The dehydrogenase activity had a strong correlation with the solids concentration. The ASBRs with 3- and 4-day cycle showed almost identical high digestion performances without adverse effect on digestion stability. The organics removals based on subnatant of the ASBRs were consistently above 90%. Remarkable increase in equivalent gas production of 52% was observed at the ASBRs compared with the control though the control and ASBRs showed similar effluent quality. Thus, digestion of a municipal sludge was possible using the ASBR in spite of high concentration of settleable solids in the sludge.