A polygalacturonase with a molecular mass of 74 kDa, an isoelectric point around pH 4.2 and pH--and temperature optima of 3.9 and 50 degrees C, respectively, was purified from a culture fluid of Penicillium frequentans. The enzyme was characterized as an exo-alpha-1,4-polygalacturonase (exo-PG I). Km and Vmax for sodium polypectate hydrolysis were 0.68 g/l and 596.8 U x mg(-1), respectively. The enzyme, a glycoprotein with a carbohydrate content of 81%, is probably the main pectinase of Penicillium frequentans responsible for cleaving monomer units from the non-reducing end of pectin.