We have evaluated the role of monofunctional heme-containing catalase encoded by cat-1 gene from the soil bacterium Comamonas terrigena N3H in the response to various forms of oxidative stress. Our results indicate that this constitutively expressed catalase represents the major source for the defence of Comamonas terrigena cells against toxic peroxides but the cells can express also a second form of catalase that is bigger and its regulation is probably more complicated. The sequence analysis confirmed the presence of highly conserved catalase sequence motifs in two environmental strains of Comamonas terrigena but in those strains that were not exposed to oxidative stress, no such sequence motif could be detected. The results obtained underline the importance of catalase expression in the defence mechanism against oxidative stress in bacterial cells.