Abstract The phototrophic bacterium Rhodobacter capsulatusphotoreduces 2,4-dinitrophenol to 2-amino-4-nitrophenol and releases stoichiometrically the product into the medium under anoxic conditions. 2,4-Dinitrophenol uptake requires low light intensities and takes place by an inducible transport system with an optimal pH of 7.0. In contrast, mononitrophenols uptake requires a pH range below their p Kvalues. Cell growth and photoreduction of 2,4-dinitrophenol by R. capsulatusdepend on the presence of alternative nitrogen sources, the highest rate being observed in the presence of N 2. 2,4-Dinitrophenol is an uncoupler that inhibits energy-requiring processes and, therefore, its uptake and metabolization by R. capsulatusallow the bacterium to grow phototrophically by fixing dinitrogen. As previously reported for nitrate uptake by R. capsulatus, the addition of the inhibitor of the glutamine synthetase l-methionine- d, l-sulfoximine prevents the dinitrophenol uptake from being inhibited by ammonium. This suggests that a process regulated by the intracellular carbon/nitrogen balance would be involved in the photoanaerobic metabolization of 2,4-dinitrophenol.