Methods for isolating mutants of Acinetobacter calcoaceticus NCIB 8250 constitutive for the mandelate enzymes were compared. Non-inducing substrates were not available. Continuous culture gave no stable constitutive mutants. Alternate culture in inducing (L-mandelate) and non-inducing (L-glutamate) media led to the isolation of mutants mesoconstitutive for L-mandelate dehydrogenase and phenylgloxylate carboxy-lyase, and hyperinducible for benzaldehyde dehydrogenase I. Screening of possible anti-inducers showed that 2-phenyl-propionate was very effective; it was a competitive inhibitor of gratuitous induction by thiophenoxyacetate, and was used for the isolation of constitutive mutants. Some of the mutants were magnoconstitutive for L-mandelate dehydrogenase, phenylglyoxylate carboxylyase and benzaldehyde dehydrogenase I and could not be further induced; others had lower specific activities which could be increased by induction with phenylglyoxylate or thiophenoxyacetate. Similar constitutive mutants were derived from a mutant which lacked L-mandelate dehydrogenase.