The amitochondriate sexually-transmitted human parasitic protozoanTrichomonas vaginalis (Bushby strain) grown anaerobically on complex medium containing cysteine and ascorbic acid consumed O2 avidly (6.9 μM min−1 per 106 organisms) with an apparentKm value of 5.1 μM O2 : O2 uptake was inhibited by O2 > 120 μM. Spectrophotometric assays in the presence of microperoxidase (419-407 nm) indicated that H2O2 was produced and that inhibition by high O2 concentrations was again evident. Hydrogenosomes oxidizing pyruvate in the presence of ADP and succinate showed similar patterns of O2 consumption, H2O2 production (33.5 pmol min−1 per mg protein), and O2 inhibition. Cytosolic NADH oxidase gave no detectable H2O2, whereas the cytosolic NADPH oxidase produced H2O2 at a rate (43 pmol min−1 per mg protein) greater than that of hydrogenosomes. These results are discussed in relation to the oxidative stress experienced by the pathogen in its natural habitat.