An endopolygalacturonase has been purified from a crude commercial preparation that is derived from cultures of Aspergillus niger. The polygalacturonase has a pH optimum of 4.5 and is relatively heat labile, losing activity when heated to 45°C. The purified preparation acts on the α‐1, 4‐galacturonosyl linkages present in polygalacturonic acid. It does not act on linkages present in a variety of substrates which contain virtually all of the glycosidic bond types described for the cell walls of monocots, dicots, and various f