Chlorate or trimethylamine-N-oxide (TMAO) added to phototrophic cultures of Rhodobacter sphaeroides DSM 158 increased both the growth rate and the growth yield although this stimulation was not observed in the presence of tungstate. This strain exhibited basal activities of nitrate, chlorate, and TMAO reductases independently of the presence of these substrates in the culture medium, and nitrate reductase (NR) activity was competitively inhibited by chlorate. Phototrophic growth of Rhodobacter capsulatus B10, a strain devoid of NR activity, was inhibited only by 100 mM chlorate. However, growth of the nitrate-assimilating R. capsulatus strains E1F1 and AD2 was sensitive to 10 mM chlorate, and their NR activities were not inhibited by chlorate. Both NR and chlorate reductase (CR) activities of strain E1F1 were induced in the presence of nitrate or chlorate respectively, whereas strain AD2 showed basal levels of these activities in the absence of the substrates. A basal TMAO reductase (TR) activity was also observed when these strains of R. capsulatus were cultured in the absence of this electron acceptor. These results suggest that chlorate and TMAO can be used as ancillary oxidants by Rhodobacter strains and that a single enzyme could be responsible for nitrate and chlorate reduction in R. sphaeroides DSM 158, whereas these reactions are catalyzed by two different enzymes in R. capsulatus E1F1 and AD2.