The effect of temperature on the reaction rate of bacteria inhabiting a microbial film was investigated based on the experimental observations of a submerged aerobic biofilter for wastewater treatment. BOD removal rate per unit surface area of packing material was not significantly affected by the decrease in temperature. The BOD removal rate was compensated with the increased number of bacterial cells eaven if cell activity was decreased. Activation energy for aerobic reaction for BOD removal and for carbon dioxide production in the microbial film was estimated to be in the range of 50-70 kJ . mol-1. This range of values coincided well with those previously reported in the literature for wastewater treatment and growth of pure bacterial culture. Moreover, it was ascertained that the production rate of carbon dioxide in the microbial film was in the range of those observed for E. coli and Salmonella sp. The production rate of carbon dioxide extrapolated from the present data using the activation energy mentioned above coincided well with the aerobic reaction rate observed in the compost of sewage sludge. Bacterial phase change in the film was studied with reference to the quinone profile as well.