Two strains of a novel species of phototrophic micro-organism were isolated from the sediments of a shallow, freshwater, eutrophic lake. Both strains grew photolithoheterotrophically with sulfide as an electron donor, transiently accumulating intracellular sulfur globules. Photolithoautotrophic growth was not observed. One strain was designated BCH(T) (the type strain) and was studied in most detail. Cells contained bacteriochlorophyll a, and the dominant carotenoid was lycopene. Cell suspensions were brown. The photosynthetic membranes had a vesicular arrangement. Acetate, propionate, pyruvate, succinate and fumarate were each used as electron donors and carbon sources in the presence of sulfide and bicarbonate. In the presence of light, growth did not occur with hydrogen, thiosulfate or iron(II). The optimum temperature for growth was between 25 and 30 degrees C, the maximum being 36 degrees C. The G+C content of the genomic DNA of strain BCH(T) was 63 mol%. Analysis of the 16S RNA genes showed that both strains belonged to the gamma-subclass of the Proteobacteria but were phylogenetically distinct from any described phototrophic organisms within the Chromatiaceae. On the basis of phylogenetic and physiological differences from other phototrophic microorganisms, strain BCH(T) is described as a novel species of a new genus, Thiobaca trueperi gen. nov., sp. nov.