1. The substrate-dependent oxygen uptake of Chromatium D has been measured. Although the rates observed were only of the order of 1 μmole/hour x mg dry weight, they are comparable with the rates found for other photosynthetic bacteria. 2. Light inhibits the rate of O2 consumption of suspensions equilibrated with excess substrate, but may stimulate the rates in previously starved suspensions. 3. Prolonged aeration of suspensions of Chromatium with thiosulfate leads to accumulation of sulfur globules in the cells, and does not lead to a rapid loss of viability. 4. Measurements of ATP content of suspensions after exposure to air indicate a limited capacity for oxidative phosphorylation in Chromatium.