Catabolic effects which exert control over the inducible synthesis of three enzymes in Arthrobacter crystallopoietes involve at least three different mechanisms: interference with inducer transport, severe catabolite repression, and transient repression. The rate of histidase induction by histidine is reduced by incubation of the cells with succinate or glucose. The maximum effect of succinate, 67% reduction in histidase production, occurs only after 100 min of incubation with succinate. At least 3h of incubation are required for the maximum effect of glucose (31% reduction in enzyme induction). Both succinate and glucose inhibit histidine transport. Cyclic adenosine 3′,5′-monophosphate (cyclic AMP), at 10-7 M, slightly stimulates the induction of histidase in cultures both with or without succinate. No conditions were found in which cyclic AMP abolishes the effect of succinate. Induction of l-serine dehydratase by glycine is severely and permanently repressed by glucose and to a lesser extent by citrate. Glucose does not affect glycine uptake. Succinate, fumarate, and aspartate, which are all better substrates than glucose or citrate for growth of A. crystallopoietes, have no effect on l-serine dehydratase induction. Induction and repression of l-serine dehydratase are not affected by cyclic AMP. Synthesis of isocitrate lyase after addition of acetate is unaffected by glucose but is severely repressed by succinate or fumarate. Aspartate and glutamate cause a transient repression of enzyme synthesis after which synthesis proceeds at the control rate. The ability to transport acetate is inducible. Development of this capacity in the presence of acetate is not affected by succinate or glutamate. Cyclic AMP has no effect on enzyme production or repression. A. crystallopoietes takes up radioactive cyclic AMP and has at least one of the enzymes of cyclic AMP metabolism, adenyl cyclase.