Oxygen is taken up by the cells of purple sulphur bacterium Thiocapsa roseopersicina BBS grown in the light under anaerobic conditions and in the darkness under aerobic conditions. Respiration is stimulated by sulphide and thiosulphate but not by organic substrates which increase the yield of the cultures. Azide and cyanide inhibit oxygen uptake by the cells in the presence of sulphide and thiosulphate, and also utilization of these compounds. Amytal, atebrin, and rotenone inhibit oxygen uptake by the cells in the presence of sulphide but have no effect on their respiration in the presence of thiosulphate. Light produces reversible inhibiting action on respiration of the cells. Therefore, Thiocapsa roseopersicina can grow in the darkness by oxidizing sulphur compounds with the participation of oxygen.