We determined the nucleotide sequence of the blaSHV-2(pBP60-1) gene from Klebsiella ozaenae which confers resistance to broad-spectrum cephalosporins. The structural gene encodes a polypeptide product of 286 amino acids, and the estimated molecular weight of the mature protein is 28,900. Amino acid sequence comparison of the SHV-2pBP60-1 enzyme with all known class A beta-lactamases and homology studies showed that the residues were highly conserved. Furthermore, SHV-2pBP60-1 was clearly related to SHV-1, LEN-1, and OHIO-1. The SHV-2pBP60-1 enzyme differed from SHV-1 isolated from Klebsiella pneumoniae by seven amino acid substitutions. One of these substitutions, the Gly----Ser substitution at position 234, is probably a key region for the novel activity of cefotaxime hydrolysis. A phylogenetic tree was constructed by using all class A beta-lactamases of known sequences by a progressive alignment method. The data suggested that the beta-lactamases of gram-positive Streptomyces, Staphylococcus, and Bacillus species appeared early in evolution, followed by the PSE and CARB enzymes of Pseudomonas species and, more recently, by the SHV-type and TEM-type enzymes found in enteric bacteria. Larger evolutionary distances separated clusters of the gram-positive beta-lactamases than separated clusters of the gram-negative enzymes. Results of this phylogenetic study suggested that extended-spectrum enzymes are recent derivatives that are selected by the use of new cephalosporins.