Abstract Preformed mycelial pads of Aspergillus niger discharge large amounts of oxalate when incubated with phosphate buffer or sodium formate. Radioactive formate becomes incorporated to a small but significant extent into oxalate, probably by way of carbonate. The latter is readily formed from formate by the organism or extracts thereof, provided the organism has beenexposed to formate during growth. This inducible and soluble enzyme has been partly purified and some of its properties studied. It carries out the oxidative decarboxylation of formate with oxygen as the only effective electron acceptor and H 2O 2 as the reaction product together with CO 2. No dialyzable cofactors participate in the reaction. The enzyme has a pH optimum of 6.2 and a Michaelis constant of 0.013 M.