An analysis of the bet genes governing the osmoregulatory choline-glycine betaine pathway of Escherichia coli was performed. A 9 kb BamHI fragment, located 30 to 39 kb counterclockwise of the EcoRI site of lacZ, coded for all known Bet activities. The following genes were identified: the betA gene for the choline dehydrogenase, the betB gene for the betaine aldehyde dehydrogenase, and the betT gene or operon for the high-affinity choline transport. The betB and the betT genes were named in this paper, and the clockwise gene order was shown to be betA,B,T. Subcloning gave plasmids which expressed each of the three Bet activities separately. The cloned bet genes remained osmotically regulated, indicating the existence of several osmotically regulated promoters in the bet region. Salmonella typhimurium, which carried the bet region of E. coli in the broad-host-range vector pRK293 expressed the three Bet activities and displayed increased osmotic tolerance in the presence of choline.