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The murB gene of Salmonella typhimurium was cloned and found to be 75% and 82% identical to the DNA and protein sequences, respectively, of the same gene in Escherichia coli. These identities are among the lowest recorded between the two bacteria. Nevertheless, wild-type S. typhimurium murB complemented the known temperature-sensitive E. coli mutant, and wild-type E. coli murB complemented three temperature-sensitive mutants of S. typhimurium. The 5S rRNA gene, rrfB, and the region between murB and rrfB were also cloned and sequenced. The rrfB gene of S. typhimurium differs from rrfB of E. coli in only 2 of 120 nt, but the region between murB and rrfB has diverged greatly and includes a sequence that elosely resembles a repetitive extragenic palindrome of the type normally associated with E. coli. Previous comparisons of gene divergence have suggested that the chromosomal mutation rate is lower in the vicinity of the origin of replication. However, the S. typhimurium murB gene, located 6 map minutes from the origin of replication, is highly substituted at synonymous sites and the sequence between murB and rrfB is significantly modified as well. Thus, murB is an exception to the general observation that genes near the origin of replication show less divergence than do genes elsewhere in the bacterial chromosome.