Tests for the capacity of nonsulfur purple bacteria to photocatabolize acetone revealed that certain strains of Rhodobacter (Rb.) capsulatus and Rhodomicribium (Rm.) vannielii could grow on this organic compound. Phototrophic growth of R. capsulatus strain B10 on acetone was CO2 dependent. Dark anaerobic or dark aerobic growth of R. capsulatus on acetone was not observed, although microaerobic growth in the dark did occur. Of a total of 13 species of nonsulfur purple bacteria examined, only strains of Rb. capsulatus and Rm. vannielii were found capable of photoheterotrophic growth on acetone.