L-Sorbose degradation in Klebsiella pneumoniae was shown to follow the pathway L-sorbose leads to L-sorbose-1-phosphate leads to D-glucitol-6-phosphate leads to D-fructose-6-phosphate. Transport and phosphorylation of L-sorbose was catalyzed by membrane-bound enzyme IIsor of the phosphoenolpyruvate-dependent carbohydrate:phosphotransferase system, specific for and regulated by this ketose and different from all other enzymes II described thus far. Two soluble enzymes, an L-sorbose-1-phosphate reductase and a D-glucitol-6-phosphate dehydrogenase, were involved in the conversion of L-sorbose-1-phosphate to D-fructose-6-phosphate. This dehydrogenase was temperature sensitive, preventing growth of wild-type strains of K. pneumoniae at temperatures above 35 degrees C in the presence of L-sorbose. The enzyme was distinct from a second D-glucitol-6-phosphate dehydrogenase involved in the metabolism of D-glucitol. The sor genes were transferred from the chromosome of nonmotile strains of K. pneumoniae by means of a new R'sor+ plasmid to motile strains of Escherichia coli K-12. Such derivatives not only showed the temperature-sensitive Sor+ phenotype characteristic for K. pneumoniae or Sor+ wild-type strains of E. coli, but also reacted positively to sorbose in chemotaxis tests.