The operational temperature of microbial fuel cell reactors influences biofilm development, and this has an impact on anodic biocatalytic activity. In this study, we compared three microbial fuel cell (MFC) reactors acclimated at 10A degrees C, 20A degrees C and 35A degrees C to investigate the effect on biomass development, methanogenesis and electrogenic activity over time. The start-up time was inversely influenced by temperature, but the amount of biomass accumulation increased with increased temperatures, the 10A degrees C, 20A degrees C and 35A degrees C acclimated biofilms resulted in 0.57, 0.82 and 5.43 g biomass (volatile suspended solids) per litre respectively at 56 weeks of operation. Biofilm build-up on the 35A degrees C anode was further demonstrated by scanning electron microscopy, which showed large aggregations of biomass accumulating on the anode when compared to 10A degrees C and 20A degrees C biofilms. Biomass accumulation had a direct impact on biocatalytic performance, with the maximum power at 35A degrees C after 60 weeks of operation being 2.14 W m(-3) and power densities for the 10A degrees C and 20A degrees C reactors being and 4.29 W m(-3). Methanogenic activity was also shown to be higher at 35A degrees C, with a rate of 10.1 mmol CH(4) biofilm per gram of volatile suspended solid (VSS) per day, compared to 0.28 mmol CH(4) per gram of VSS per day produced at 20A degrees C. These results demonstrate that higher MFC operating temperatures could be detrimental to the biocatalytic performance of electrochemically active bacteria in anodic biofilms due to biomass accumulation with enhanced development of non-electrogenic communities (e.g. methanogens and fermenters), meaning that, over time, psychro- or mesophilic operation can have beneficial effects for the development of electrogenically active populations in the reactor.