A culture of phototrophic purple bacteria containing bacteriochlorophyll b has been isolated. The bacterium forms a sessile bud and, along with producing motile swarmer cells, it produces immobile oval cells surrounded with a slime capsule. It grows on media with simple organic substances in the presence of yeast extract or vitamins, under anaerobic conditions in the light or under microaerophilic conditions in the darkness. Besides organic substances, the culture assimilates hydrogen sulphide and thiosulphate as electron donors; it is incapable of assimilative sulphate reduction. In the light, the bacterium oxidizes thiosulphate to sulphates, without accumulation of molecular sulphur. The bacterium is classed as a new species of the genus Rhodopseudomonas--Rhodopseudomonas sulfoviridis nov. sp.