The structural gene of a carbenicillinase was cloned from the chromosomal DNA of Proteus mirabilis GN79. This gene codes for a protein of 270 amino acids. Alignment of the amino acid sequence with those of known beta-lactamases revealed that the enzyme is a novel class A beta-lactamase with a unique conserved triad, RTG. By using a DNA fragment of the structural gene, a lack of cross hybridization was confirmed between the DNA probe and total DNAs from natural isolates of P. mirabilis, suggesting that the carbenicillinase may not be a species-specific beta-lactamase of P. mirabilis.