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Two distinct exo-polygalacturonases (exo-PGs) have been obtained from the culture filtrate of Aspergillus niger. The two exo-PGs were purified by CM-Sephadex C-50 column chromatography, Sephadex G-100 gel filtration, DEAE-Sephadex C-50 column chromatography and Sephadex G-75 gel filtration. The exo-PG I and II were purified approximately 15-and 25-fold, and they were found to have Km values of 20 and 3.85mg/ml, respectively, from the LINEWEAVER-BURK plot. The molecular weights of exo-PG I and II were 66000 and 63000, and had an isoelectric point of pH 5.6 and 3.8, respectively.The pH optima for exo-PG I and II were at pH 3.8 and 4.8. However, pH stability, optimum temperature and thermal stability of these enzymes were almost similar. The interesting feature of these enzymes is the activation by mercury ions. HgCl2 greatly activated exo-PG I but not exo-PG II.