We conducted several field surveys on periphyton biofilm in a local river in the basin of which a sewage treatment system just began to spread. We investigated the development of the biofilm at different points along the Hinuma River. The periphyton biofilm were used to determine the nitrification activity which would be an overall index of nitrogenous pollution. The field survey showed that the periphyton density gradually increased to the maximum level around 2 months, if there were not significant change of river flow. In the nitrification tests, supply of ammonium nitrogen from the biofilms itselfwas observed. Therefore, nitrification activity was evaluated considering the degradation of attached algae as a biofilm component. The activity was higher for biofilms in the urban drainage receiving domestic wastewater than that at the upstream of the river which was less polluted. The biofilm receiving secondary effluent had little nitrification activity. This indicated that residual chlorine or by-product of chlorine disinfection might suppress the nitrification in the biofilm.