Aim: The aim of this study was to determine the relationship between reactive oxygen species (ROS) production and conidial infectivity in Beauveria bassiana. Methods and Results: Beauveria bassiana Bb 882.5 was cultured in solid-state culture (SSC) using rice under three oxygen conditions (21%, or pulses at 16 and 26%). Hydrophobicity was determined using exclusion phase assay. Bioassays with larvae or adults of Tenebrio molitor allowed the measurements of infectivity parameters. A fluorometric method was used for ROS quantification (superoxide and total peroxides). NADPH oxidase (NOX) activity was determined by specific inhibition. Conidial hydrophobicity decreased by O-2 pulses. Mortality of larvae was only achieved with conidia harvested from cultures under 21% O-2; whereas for adult insects, the infectivity parameters deteriorated in conidia obtained after pulses at 16 and 26% O-2. At day 7, ROS production increased after 16 and 26% O-2 treatments. NOX activity induced ROS production at early stages of the culture. Conclusion: Modification of atmospheric oxygen increases ROS production, reducing conidial quality and infectivity. Significance and Impact of the Study: This is the first study in which conidial infectivity and ROS production in B. bassiana has been related, enhancing the knowledge of the effect of O-2 pulses in B. bassiana.