Catalase activities in crude extracts of exponential and stationary phase cultures of various bacteria were visualized following gel electrophoresis for comparison with the enzymes fromEscherichia coli.Citrobacter freundii,Edwardsiella tarda,Enterobacter aerogenes,Klebsiella pneumoniae, andSalmonella typhimuriumexhibited patterns of catalase activity similar toE.coli, including bifunctional HPI-like bands and a monofunctional HPII-like band.Proteus mirabilis,Erwinia carotovora, andSerratia marcescenscontained a single band of monofunctional catalase with a mobility intermediate between the HPI-like and HPII-like bands. The cloned genes for catalases HPI (katG) and HPII (katE) fromE.coliwere used as probes in Southern hybridization analyses for homologous sequences in genomic DNA of the same bacteria.katGwas found to hybridize with fragments fromC.freundii,Ent.aerogenes,Sal.typhimurium, andK.pneumoniaebut not at all withEd.tarda,P.mirabilis,S.marcesens, orEr.carotovora.katEhydridized withC.freundiiandK.pneumoniaeDNAs and not with the other bacterial DNAs.Key words: catalase genes, bacteria, homology.