The cobA gene of Salmonella typhimurium was cloned, sequenced and overexpressed. A 990-bp HpaI-SacI fragment was cloned into the multiple cloning site of plasmid pSU19, an intermediate-copy-number vector. DNA sequence analysis established that cobA is 588 bp in length and codes for a protein with a predicted molecular weight of 21.7 kDa. However, the CobA protein expressed from the T7 promoter migrated as a 25-kDa protein on SDS-polyacrylamide gels. A high degree of identity at the amino acid sequence level was established between the CobA, Pseudomonas denitrificans CobO and Escherichia coli BtuR proteins. P. denitrificans CobO has been shown to be a ATP:corrinoid adenosyltransferase enzyme. Based on the similarities between CobO and CobA, and the phenotypes of cobA mutants, we suggest that CobA is the ATP:corrinoid adenosyltransferase of S. typhimurium.