The catalase gene, vktA, of the hydrogen peroxide-resistant bacterium Vibrio rumoiensis S-l has been isolated and sequenced. Plasmid pBSsa1 was obtained by genome Library screening and it complemented a catalase-less mutant, Escherichia coli strain UM2, indicating that pBSsa1 contained the catalase gene (vktA). The vktA gene consisted of an open reading frame of 1530 bp encoding a 508 amino-acid protein with a calculated size of 57657.79 Da. The deduced amino acid sequence showed high homology with that of bacterial group Ln catalases. Investigation of the subcellular catalase localization using an immuno-electron microscopical technique revealed that a larger amount of VktA catalase is present in the periplasmic space than in the cytoplasm. The periplasmic space of this bacterium can therefore be regarded as a reservoir of VktA catalase in V. rumoiensis S-1.