It was previously suggested that a population of phototrophic bacteria, Rhodopseudomonas palustris strain RN1 and Blastochloris sulfoviridis strain GN1 could be induced from granules in a lighted upflow anaerobic sludge blanket (LUASB) reactor. The present study showed that both strains RN1 and GN1 could use acetate, propionate, butyrate, and lactate as electron donors under anaerobic light conditions. The composition of organic acids in the effluent from the LUASB reactor was studied to investigate competitive consumption between acetogenic bacteria, methanogens, and phototrophic bacteria in the reactor. When acetate, propionate and lactate were supplied to the reactor, a small amount of acetate and propionate was observed in the effluent under light conditions. The concentrations of acetate and propionate increased under dark conditions compared with those under light conditions using organic acid and peptone media as the influent. When starch was supplied to the reactor, the concentrations of formate, acetate, propionate, butyrate, and lactate in the effluent were less than 0.5 mg C.l(-1) during operation under light and dark conditions. The concentrations of ammonium and phosphate in the effluent under dark conditions were higher than those under Light conditions. These results suggested that phototrophic bacteria in the LUASB reactor consumed acetate and propionate as well as ammonium and phosphate in competition with methanogens and acetogenic bacteria.