Hydrogen peroxide (H2O2) inhibits microbial growth at a specific concentration. However, we previously isolated two environmental bacterial strains that exhibited sensitivity to a lower H2O2concentration in agar plates. Putative catalase genes, which degrade H2O2, were detected in their genomes. We herein elucidated the characteristics of these putative genes and their products using a self-cloning technique. The products of the cloned genes were identified as functional catalases. The up-regulation of their expression increased the colony-forming ability of host cells under H2O2pressure. The present results demonstrated high sensitivity to H2O2even in microbes possessing functional catalase genes.