In the presence of nitroaromatic and haloaromatic derivatives, Rhodobacter capsulatus E1F1 growth was affected in different degrees depending on the nitrogen source used. Phototrophic growth on glutamate or ammonium was not inhibited by 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP), 2-amino-4-nitrophenol (2,4-ANP), 4-aminophenol (4-AP), or 4-chlorophenol (4-ClP), whereas 2,4-dinitrophenol and 4-chlorophenol partially inhibited bacterial growth in nitrate, nitrite, and dinitrogen. On the other hand, although photosynthetic nitrate uptake was significantly inhibited by 2,4-dinitrophenol, 4-chlorophenol inhibited it to a lesser extent. Nitrogen fixation was severely inactivated in vivo by 2,4-dinitrophenol, but nitrate reductase activity was activated in vivo by 2,4-dinitrophenol, 4-nitrophenol, and 4-chlorophenol. Similar effects were found in cells growing with nitrate and 2,4-dinitrophenol under dark and aerobiosis conditions. None of the enzymatic activities related to inorganic nitrogen assimilation were affected by xenobiotics in vitro.