The interaction between phototrophic dinitrogen fixers and methanogens was examined in soil slurries amended with rice straw using 2-bromoethanesulfonic acid (BES), a specific methanogenic inhibitor. Slurries incubated in light increased phototrophic nitrogenase activity (acetylene reducing activity), and showed growth of phototrophic purple bacteria and reduction of CH4 emission, indicating outcompetition of purple bacteria with methanogens in photic zones. Adding BES effectively inhibited methane production and markedly increased phototrophic acetylene reducing activity accompanied with acetate accumulation, but did not affect populations of purple bacteria in the slurries. More acetate accumulated in the inhibited slurries incubated in dark. We suggest that increased availability of organic substrates for purple bacteria after stopping methanogenic consumption by BES caused the increased phototrophic acetylene reducing activity. These results indicate that, after purple bacteria grow enough, performance of their N2 fixation may be limited by substrate availability, which methanogenesis may profoundly influence.