The inhibitory effects of 2-hydroxybiphenyl on various electron transport reactions of isolated membranes and growth in the presence of malate of either phototrophic or chemotrophic cells of Rhodospirillum rubrum were studied. 50 % inhibition of both oxygen uptake of whole cells and growth under chemotrophic conditions (i.e. aerobiosis in the dark) was achieved in the presence of 0.09 mM 2-hydroxybiphenyl. With isolated membranes the same effect on NADH oxidasc was obtained with 0.08mM of inhibitor. Succinate dependent respiratory reactions were inhibited by 50% at a concentration of 0.36raM. Growth under phototrophic conditions (i.e. anaerobiosis in the light) was inhibited by 50~ in the presence of 0.17mM (wild type strain) or 0.21mM (blue-green mutant, strainVI) of 2-hydroxybiphenyl. Photophosphorylation and light dependent NAD + reduction by succinate were inhibited by 50% at concentrations of 0.21 mM and 0.03 mM of inhibitor, respectively. After phototrophic growth of the organisms for about five doublings of cell mass in the presence of 0.18raM of 2-hydroxybiphenyl coloured earotenoids could no longer be detected. Membrane fractions of such cultures exhibited normal activities of succinate cytochromec reductase but activities of NADH cytochromec reductase were decreased by 80 ~. In comparison with a blue green mutant, strain V1, of R. rubrum light induced absorbance changes at 865 nm as well as activities of photophosphorylation were unaffected. However, no activity of light dependent NAD + reduction with succinate could be detected. The data indicate that cellular respiration as well as chemotrophic growth depend largely on NADH dependent respiration. Phototrophic growth, on the other hand, is limited by photophosphorylation while Abbreviation. BChl = bacteriochlorophyll energy dependent reversed electron flow to NAD +, if at all, is of rather minor importance.