Rice straw was added to a sewage sludge digester and its effects on methane production, dewatering characteristics, and microbial communities in the digested sludge were examined by a continuous digestion experiment under mesophilic conditions (35 °C). Stable gas generation was monitored in all digestion experiments. Methane yield from raw sludge, chopped rice straw and softened rice straw were estimated to be 0.27, 0.18 and 0.26 NL/g total solids load, respectively. The capillary suction time of digested sludge was decreased by the addition of rice straw. Archaeal and bacterial communities in the sludge were elucidated by PCR-DGGE (polymerase chain reaction--denaturing gradient gel electrophoresis) targeting 16S rRNA genes. The Shannon index of DGGE profiles indicated that bacterial diversity increased with the addition of softened rice straw. DNA sequences of significant bands of the digested sludge were most closely related to Methanosaeta concilii (97.4% identity) and Methanoculleus bourgensis (100% identity). Meanwhile, those in the co-digested sludge with rice straw were most closely related to Methanosarcina barkeri (98.4% identity) and Methanoculleus bourgensis (99.3% identity). Although both Methanosaeta spp. and Methanosarcina spp. metabolize acetate to methane, Methanosarcina spp. have a competitive advantage at acetate concentrations of >70 mg/L. Results suggested that the quantity of acetate produced during rice straw degradation may change the archaeal community.