A strain ofPseudomonaswas grown with phenylacetic acid as the carbon source in batch, continuous, and phased cultures. The cellular activities of the inducible enzymes involved in the sequential degradation of phenylacetic acid [phenylacetic acid →p-hydroxyphenylacetic acid →3,4-dihydroxyphenylacetic acid →δ-carboxymethyl-α-hydroxymuconic semialdehyde] varied with the above methods of culture, and illustrate microbial reaction to environment (10). The effect of substrate concentration on the activity of the enzymes in the metabolic breakdown of phenylacetic acid is discussed.The changes observed in the cells show some of the disadvantages inherent in batch cultivation methods for the production of certain enzymes and some of the relative advantages in chemostat and continuous phased cultures for obtaining higher yields.