Pseudomonas putida, a saprophytic root-colonizing bacterium, produces multiple forms of catalase. Catalase A, which increases in specific activity during growth phase and after treatment with H2O2, is located in the cytoplasm and is inhibited by 3-amino-1,2,4-triazole, EDTA, and cyanide, but not by chloroform-methanol treatment. Catalase B, which is induced by external H2O2 or during stationary phase of growth, is membrane associated and is inhibited by chloroform-methanol, EDTA, and cyanide, but not by aminotriazole. Catalase A has a broad pH optimum, from pH 6.0 to 11.0, with two peaks, at pH 8.0 and 11.0. Catalase B is most active at pH 5.0-11.0 Mutant J-1, generated by ethyl methanesulfonate mutagenesis, lacked catalase A activty in extracts of cells harvested throughout lag to early stationary growth phase in liquid medium. Catalase B was produced by J-1 in stationary phase. Exposure of J-1 to H2O2 caused the production of both catalase A and catalase B. Mutant J-1 was more susceptible to cell death than the wild type upon direct exposure to 2.5 mM H2O2 but survived the treatment after exposure to lower (0.3 mM), nonlethal doses of H2O2. The ability to adapt to H2O2 may be related to the behavior of J-1 on roots where active oxygen species are produced by root surface enzymes. J-1 colonized root surfaces at wild-type levels and produced catalases A and B after exposure to root surfaces for 12 h.