The capability of five strains of the phototrophic bacterium Rhodopseudomonas palustris to produce molecular hydrogen (Hz) from the aromatic acids benzoate, p-hydroxybenzoate, cinnamate and D- and L-mandelate was investigated. Optimal Hz production was achieved when the strains were grown anaerobically in the light at 10,000 lx under nitrogen (N) limitation using 1 mM L-glutamate as an N source. In the presence of 2 mM benzoate or L-mandeIate as carbon and electron sources, strain DSM 131 produced 45% H-2 Of the maximal theoretical value and strain F2 32%, respectively. Increased H-2 production correlated with increased nitrogenase activities, but HZ formation was not further stimulated by inhibition of the Hz uptake (hup) hydrogenase with ethylenediaminetetraacetic acid (EDTA).