Twenty-one Xyl- mutants of Salmonella typhimurium were selected: all had lost one or more of the activities for D-xylose isomerase, C-xylulokinase, or D-xylose transport. The mutants were classified into five functional groups: xylR, pleiotropic negative (12 mutants); xylA, D-xylose isomerase defective (3 mutants); xylB, D-xylulokinase defective (2 mutants); xylT, D-xylose transport defective (1 mutant); and 3 mutants with defective D-xylose isomerase and D-xylulokinase. Some nonsense mutations were identified among the xylR mutants. Two F'xyl plasmids were isolated by selection for early transfer of xyl+ by an Hfr which transfers xyl as a terminal gene; a plasmid with a mutation in the xyl genes, F'xylR1, was also isolated. Complementation tests using F'xyl plasmids indicate that expression of the xylA, xylB, and xylT genes is under the positive control of the xylR regulatory gene. Conjugation crosses and P22-mediated transduction data indicate that all the xyl mutations tested are in a cluster of genes at 78 units on the linkage map, and that the gene order is xylT--xylR--xylB--xylA--glyS--mtlA,D.