Obligate anaerobes are periodically exposed to oxygen, and it has been conjectured that on such occasions their low-potential biochemistry will predispose them to rapid ROS formation. We sought to identify scavenging enzymes that might protect the anaerobe Bacteroides thetaiotaomicron from the H2 O2 that would be formed. Genetic analysis of eight candidate enzymes revealed that four of these scavenge H2 O2  in vivo: rubrerythrins 1 and 2, AhpCF, and catalase E. The rubrerythrins served as key peroxidases under anoxic conditions. However, they quickly lost activity upon aeration, and AhpCF and catalase were induced to compensate. The AhpCF is an NADH peroxidase that effectively degraded low micromolar levels of H2 O2 , while the catalytic cycle of catalase enabled it to quickly degrade higher concentrations that might arise from exogenous sources. Using a non-scavenging mutant we verified that endogenous H2 O2 formation was much higher in aerated B. thetaiotaomicron than in Escherichia coli. Indeed, the OxyR stress response to H2 O2 was induced when B. thetaiotaomicron was aerated, and in that circumstance this response was necessary to forestall cell death. Thus aeration is a serious threat for this obligate anaerobe, and to cope it employs a set of defences that includes a repertoire of complementary scavenging enzymes.