2-Pyrone-4,6-dicarboxylate hydrolase was purified from 4-hydroxybenzoate-grown Pseudomonas testosteroni. Gel filtration and electrophoretic measurements indicated that the preparation was homogeneous and gave a molecular weight of 37,200 for the single subunit of the enzyme. Hydrolytic activity was dependent upon a functioning sulfhydryl group(s) and was freely reversible; the equilibrium position was dependent upon pH, with equimolar amounts of pyrone and open-chain form present at pH 7.9. Since the hydrolase was strongly induced when the nonfluorescent organisms P. testosteroni and P. acidovorans grew with 4-hydroxybenzoate, it is suggested that 2-pyrone-4,6-dicarboxylate is a normal intermediate in the meta fission degradative pathway of protocatechuate. Laboratory strains of fluorescent pseudomonads did not metabolize 2-pyrone-4,6-dicarboxylate, but a strain of P. putida was isolated from soil that utilized this compound for growth; the hydrolase was then induced, but it was absent from extracts of 4-hydroxybenzoate-grown cells that readily catabolized protocatechuate by ortho fission reactions. 2-Pyrone-4,6-dicarboxylic acid was the major product formed when gallic acid was oxidized by purified protocatechuate 3,4-dioxygenase. Protocatechuate 4,5-dioxygenase gave only the open-chain ring fission product when gallic acid was oxidized, but the enzyme attacked 3-O-methylgallic acid, giving 2-pyrone-4,6-dicarboxylic acid as the major product. Cell suspensions of 4-hydroxybenzoate-grown P. testosteroni readily oxidized 3-O-methylgallate with accumulation of methanol.