Oxalic acid production and pectolytic enzyme activity were detected in inoculated onion bulb tissues by Aspergillus niger. A chain-splitting enzyme, which reduced the viscosity of polygalacturonate, from inoculated onion bulb tissues was purified 22-fold by ammonium sulfate fractionation, SP-Sephadex column chromatography and gel filtration of Sephadex G-100. The purified enzyme formed a single protein band on polyacrylamide gel electrophoresis. Among 7 plant species tested, onion tissue was the most macerated by the enzyme itself. The optimum pH for the enzyme activity was 3.0 on onion tissues and the enzyme lost activity at 70C for 10min. Oxalic acid production and macerating activity in inoculated onion bulb tissues increased progressively with incubation time. Oxalic acid was as much as 6.6mg/g fresh weight in inoculated onion bulb tissues 10 days after incubation and pH of the onion tissues dropped from 6.5 to 1.9. A possible synergistic role for each of these constituents produced in rot of onion bulbs is indicated by our results that lowering of pH of the bulb tissues by oxalic acid provided the activity of the enzyme with the optimum pH value in the bulb tissues.