The role of adenine nucleotides in the control of the energy metabolism of Chromatium has been studied through the measurement of the levels of ATP ADP, and AMP in growing cultures. Comparison of these levels with the cellular chlorophyll levels indicates that ATP concentration, and not those of ADP, AMP, or any function of concentration of these three nucleotides, is a controlling factor in chlorophyll synthesis. In addition, the relationship of cellular ATP and chlorophyll levels to sulfur metabolism furnishes further evidence for the existence of two photosystems, postulated previously. The effects of high levels of exogenous ATP and ADP are consistent with these findings.