The extraordinarily high level of H2O2 tolerance of Vibrio rumoiensis strain S-1(T) when compared with the tolerance levels of strain S-4, a probable catalase-deficient derivative of strain S-1(T), was demonstrated by the introduction of 0-100 mM H2O2 during the mid-exponential growth phase. The contribution of catalase to the H2O2 tolerance was also demonstrated by comparing the catalase-deficient mutant Escherichia coli strain UM2 with a UM2 strain, harboring the plasmid pBSsal, which carried the strain S-1(T) catalase gene vktA. The decomposition rates of 2325 mM H2O2 that was introduced in the culture fluids of strain S-1(T) and E. coli UM2 harboring pBSsal corresponded to the calatase activities of the cells by spectrophotometric measurements. The presence of cell surface catalase was observed by immunoelectron microscopy, using an antibody for intracellular catalase in strain S-1(T). The high level of H2O2 tolerance of strain S-1(T) was attributable to the catalase activity of the cells. Cell surface catalase is considered to contribute to the catalase activity of strain S-1(T) cells.