Growth of phototrophic bacteria was induced from granules in a lighted upflow anaerobic sludge blanket (LUASB) reactor supplied with an organic-acid-based medium containing 141.7 mg S l(-1) of SO42- under light conditions (100 muE .m(-2).s(-l)). We investigated the population dynamics of phototrophic bacteria in the LUASB reactor and the performance of the LUASB reactor for wastewater treatment and poly-beta -hydroxybutyrate (PHB) production under anaerobic light and sulfate-rich conditions. In vivo absorption spectra and a colony count suggested that populations of Rhodopseudomonas palustris and Blastochloris sulfoviridis in the LUASB reactor supplied with a medium containing 574.4 mg S .l(-1) of SO42- under light conditions were lower than those supplied with a medium containing 1.0 or 141.7 mg S .l(-1) of SO42- under parallel conditions. Removal efficiencies of ammonium and phosphate in the LUASB reactor supplied with the medium containing 141.7 mg S .l(-1) of SO42- under light conditions were higher than those under parallel conditions but without illumination. The difference in the results of runs under light or dark conditions suggested that the ammonium and phosphate ion removal efficiencies were improved by increasing the amount of phototrophic bacterial biomass in the LUASB reactor under sulfate-rich conditions. The average PHB production rates of the bacterial cells recovered from the effluent of the LUASB reactor supplied with a medium containing 141.7, 283.5 or 574.4 mg S .l(-1) of SO42- were 1.0-2.9 mg .l(-1)-reactor d(-1) and the average PHB content based on the dry bacterial biomass was 1.4-3.6%.