We isolated spontaneous and transposon insertion mutants of Escherichia coli K-12 that were specifically defective in utilization or in high-affinity transport of D-ribose (or in both). Cotransduction studies located all of the mutations near ilv, at the same position as previously identified mutations causing defects in ribokinase ( rbsK ) or ribose transport ( rbsP ). Plasmids that complemented the rbs mutations were isolated from the collection of ColE1 hybrid plasmids constructed by Clarke and Carbon. Analysis of those plasmids as well as of fragments cloned into pBR322 and pACYC184 allowed definition of the rbs region. Products of rbs genes were identified by examination of the proteins produced in minicells containing various rbs plasmids. We identified four rbs genes: rbsB , which codes for the 29-kilodalton ribose-binding protein; rbsK , which codes for the 34-kilodalton ribokinase ; rbsA , which codes for a 50-kilodalton protein required for high-affinity transport; and rbsC , which codes for a 27-kilodalton protein likely to be a transport system component. Our studies showed that these genes are transcribed from a common promoter in the order rbsA rbsC rbsB rbsK . It appears that the high-affinity transport system for ribose consists of the three components, ribose-binding protein, the 50-kilodalton RbsA protein, and the 27-kilodalton RbsC protein, although a fourth, unidentified component could exist. Mutants defective in this transport system, but normal for ribokinase , are able to grow normally on high concentrations of the sugar, indicating that there is at least a second, low-affinity transport system for ribose in E. coli K-12.