The effect of three levels of oxygen (normal atmosphere (21% O(2)), low oxygen (16% O(2)) and enriched oxygen (26% O(2))) on the production and germination of conidia by Beauveria bassiana was evaluated using rice as a substrate. The maximum yield of conidia was achieved under hypoxia (16% O(2)) after 8 days of culture (1.51 × 10(9) conidia per gram of initial dry substrate), representing an increase of 32% compared to the normal atmosphere. However, germination was reduced by at least 27% due to atmospheric modifications. Comparison of antioxidant enzyme activity (superoxide dismutases and catalases) with the oxidation profiles of biomolecules (proteins and lipids) showed that a decrease in catalase activity in the final days of culture coincided with an increase in the amount of oxidized lipids, showing that oxidative stress was a consequence of pulses of different concentrations of O(2). This is the first study describing oxidative stress induction by atmospheric modification, with practical implications for conidia production.