The incubation of Aerobacter aerogenes PRL-R3 with ribitol resulted in the induction of ribitol dehydrogenase and d-ribulokinase, coordinately controlled enzymes of the pathway of ribitol catabolism. A dehydrogenase-negative mutant was unable to induce d-ribulokinase activity following incubation with ribitol. Similar experiments using a kinase-negative mutant resulted in normal induction of ribitol dehydrogenase, as compared to the wild-type PRL-R3 strain. Constitutive or induced cells for l-fucose isomerase were capable of catalyzing the isomerization of d-arabinose to d-ribulose. In contrast to the experiments using ribitol as the substrate, the isomerization of d-arabinose resulted in the induction of d-ribulokinase with dehydrogenase-negative cells. These data indicated that d-ribulose, rather than ribitol, acts as the inducer of the enzymes for ribitol degradation.