Strain PM-8 of Rhodopseudomonas spheroides is unable to grow photosynthetically even though it generates a normal complement of bacteriochlorophyll and carotenoids when grown aerobically in the dark. This mutant strain fails to show any of the light reactions, observable in the photosynthetically competent parent strain, that signal the operation of photochemical reaction centers. These light-induced effects include absorbancy changes, fluorescence transients, and an electron-spin-resonance signal. The pigment P870 is missing in strain PM-8.Bacterial chromatophores have been observed to catalyze a variety of photochemical electron-transfer reactions involving externally added substances such as cytochrome and quinones. Chromatophores from strain PM-8 prove to be inert or extremely inefficient in catalyzing the following reactions: (I) oxidation of reduced tetramethylphenylenediamine or of mammalian cytochrome c coupled with reduction of an endogenous substance; (2) oxidationi of reduced phenazine methosulfate or cytochrome c coupled with reduction of the ubiquinones UQ2 and UQ6. The same reactions are catalyzed efficiently (with quantum requirements of the order of 1–3 quanta per electron transfer) by chromatophores from photosynthetically competent strains of R. spheroides, even when the major bacteriochlorophyll component of the chromatographores has been converted to bacteriopheophytin.