1. Two strains of Pseudomonas were grown with phenol and used to prepare cell extracts that metabolized catechol with the transient formation of 2-hydroxymuconic semialdehyde. 2. One of these preparations catalysed the conversion of 1mol. of catechol into 1mol. each of formate and 4-hydroxy-2-oxovalerate. 3. A method for the determination of 4-hydroxy-2-oxovalerate is described, together with some properties of this compound and its 2,4-dinitrophenylhydrazone. 4. Another partially purified cell extract converted 1mol. of 4-hydroxy-2-oxovalerate, formed enzymically from catechol, into 1mol. each of acetaldehyde and pyruvate. This aldolase had a pH optimum of about 8.8, was stimulated by Mg(2+) ions and appeared to attack only one enantiomer of synthetic 4-hydroxy-2-oxovalerate.