An investigation was made into the ecological factors which influence the growth of the purple sulfur bacteria, particularly Chromatium and Thiopedia. By utilizing radioactive sulfide it was indicated that sulfide uptake is enzymatic in nature. Fructose, glucose, acetate, pyruvate, and propionate were shown to be capable of supplying electrons for the metabolism of Thiopedia. Fructose, glucose, acetate, pyruvate, lactate, formate, and propionate were able to supply electrons for the metabolism of Chromatium. These organic compounds with the exception of pyruvate were not capable of supplying carbon for synthesis of new cell material. In a field study made upon waste lagoons, microstratiflication was demonstrated to be of great importance in maintaining semi-anaerobic conditions necessary for the growth of the purple sulfur bacteria. (Modified author abstract)