Rhodobacter capsulatus E1F1, a phototrophic purple nonsulfur bacterium capable of photoassimilating nitrate or nitrite, grew phototrophically in the presence of mono- and dinitrophenols with acetate as a carbon source, the highest growth levels being obtained under microaerobic conditions. Utilization of 2,4-dinitrophenol was strictly light dependent, was inhibited by O2 and by ammonium, and took place with the simultaneous and stoichiometric production of 2-amino-4-nitrophenol, which accumulated in the medium and was poorly used for further growth in anaerobiosis. Metabolism of mononitrophenols was also light dependent but was activated by O2 and by ammonium. Metabolism of nitrophenols seemed to depend on inducible systems which were repressed in nitrogen-starved cells. Induction of the in vivo 2,4-dinitrophenol reducing system was strongly inhibited by chloramphenicol.