Nonsulfur purple photosynthetic bacteria generally possess the adaptive capacity to grow anaerobically (photosynthetically) and aerobically in darkness (energy obtained by respiratory phosphorylation). To develop a test system for study of interactions between the photosynthetic energy-conversion and dark respiration systems and the mechanism by which O2 inhibits bacteriochlorophyll synthesis, we have isolated a mutant of Rhodopseudomonas capsulata that is incapable of dark aerobic growth. Both the parent strain and the mutant strain Z-1 (aer-13) grow readily anaerobically in the light with either malate or succinate as the source of carbon and reducing power. They differ, however, in that membrane fragments (“chromatophores”) from the mutant show severely impaired ability to use reduced nicotinamideadenine dinucleotide (NADH) as a source of electrons for respiration, oxidative phosphorylation, or “redox buffering” of the photophosphorylation system. The results presented indicate that the genetic lesion responsible for greatly diminished capacity to oxidize NADH can account for inability of Rps. capsulata Z-1 (aer-13) to grow aerobically in darkness.