A Pseudomonas species adapted to grow on phenylacetic acid is simultaneously adapted to the utilization of phenylacetic acid, p-hydroxyphenylacetic acid, and 3,4-dihydroxyphenylacetic acid. Extracts of the organism catalyze the oxidation of p-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid, but not phenylacetic acid. The addition of NAD or NADH to extracts is required for maximum utilization of p-hydroxyphenylacetic acid; ferrous ion is required for maximum utilization of 3,4-dihydroxyphenylacetic acid. The oxidation of 3,4-dihydroxyphenylacetic acid results in the production of a compound having an absorption maximum at 318 mμ in acid and at 380 mμ in alkali, which was identified as δ-carboxymethyl-α-hydroxymuconic semialdehyde. The degradation of phenylacetic acid by this organism follows a pathway that utilizes p-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid as intermediates.