H2S dissolved in water can be converted to elementary sulfur or sulfate by the photosynthetic bacterium Chlorobium thiosulfatophilum. The effects of the light/dark cycle on cell growth and the rate of sulfide removal were investigated to develop an appropriate fermentation strategy. Dark fermentation was also studied without addition of H2S and CO2 as electron and carbon sources. Average specific growth rates of bacterial cultures with a continuous supply of H2S and CO2 both in light and dark conditions were occurred in the range of 0.008 to 0.009 h(-1), indicating little dependence on the light/dark cycle, but about 25% of the growth rate that was occurred only in the presence of light. Average H2S removal capacities for cultures grown under the Light/dark cycles of 14/10, 12/12, and 9/15 h, respectively, with a continuous supply of feed gases, were 0.08, 0.07, and 0.04 mu mol H2S.min(-1)/mg protein.l(-1) in the dark, and was slightly less than those in the light. H2S removal capacity with variation of the light/dark cycle was about 30-60% of that obtained in the continuously illuminated cultures. ATP concentration in the dark decreased from 0.43 to 0.37 mg ATP.mg protein(-1) as the daily dark duration decreased from 15 to 10 h. The production rate for lactic acid from a culture grown without a supply of mixtures of H2S and CO2 gases was 0.218 g Lactic acid.l(-1).h(-1), much more than that grown with a supply of feed gas mixtures. Time-averaged concentrations of lactic acid produced overall during the light and dark periods were 13.7 g lactic acid.l(-1) during the light/dark cycle of 14/10 h without a supply of feed gas, and 3.1 and 2.4 g lactic acid.l(-1) during the cycles of 9/15 and 14/10 h, respectively, with a supply of feed gas.