|Abstract||Bacillus anthracis is a gram-positive, spore-forming bacterium and the causative agent of the deadly disease anthrax. The B. anthracis genome consists of chromosomal genes and the pX01 and pX02 plasmids that strongly contribute to the bacteria's deadly nature. While the virulence factors associated with the plasmids have been extensively studied, we believe there are still undiscovered chromosomal genes that may also have important virulence factors. To identify novel chromosomal genes associated with B. anthracis virulence, we screened a transposon mutant library of B. anthracis Sterne strain for increased sensitivity to reactive oxygen species. Reactive oxygen species such as hydrogen peroxide (H2O2) have many functions in mammalian immune defenses and wild type B. anthracis is able to subvert this host defense. Sensitivity to reactive oxygen species was tested through in vitro H2O2 assays and after several rounds of screening seven mutants were confirmed as susceptible. We next tested whether any of these mutants were attenuated in vivo using our invertebrate animal model, Galleria mellonella, and found several mutants with decreased virulence. We have identified the location the transposon insertion in one of these mutants. High priority for future research will be to determine the sites of disruption of the other promising mutants. This could lead to the discovery of novel B. anthracis virulence genes and eventually possible treatment targets for future anthrax outbreaks and attacks.