An exopolygalacturonase produced by Fusarium oxysporum f. sp. radicis lycopersici, a fungus that produces root rot, was purified by gel filtration and ion exchange chromatography. It had a Mr 68 K, a pH optimum of 5.6 and an optimum temperature of 60°C. This polygalacturonase was inhibited by calcium ions and had a Km of 0.64 mM using sodium polypectate as substrate. The exo mode of action of this enzyme was revealed by thin‐layer chromatography of hydrolysed substrate.