m-Hydroxybenzoic acid 4-hydroxylase was isolated and partially purified from Aspergillus niger grown in presence of m-hydroxybenzoic acid. The enzyme catalyzed the stoichiometric formation of protocatechuic acid (3,4-dihydroxybenzoic acid) from m-hydroxybenzoic acid with the consumption of NADPH and molecular oxygen. The reaction proceeded best at pH 7.2 and showed a requirement for FAD.