Abstract: Tomato fruits infected with Penicillium funiculosum Thom. produced proteins which showed appreciable polygalacturonase activity within eight days. Uninfected tomato fruits showed only traces of polygalacturonase activity. The enzyme was partially purified by a combination of gel filtration and ion-exchange chromatography. Two components with molecular weight estimates of approximately 223,800 daltons and 89,100 daltons were expressed. Only the components of the lighter peak showed polygalacturonase activity. The enzyme showed optimum activity at pH 4.5 and 40°C. It possessed an apparent Km of 0.05 mg mL-1 for the hydrolysis of pectin. Na+ and CaH ions were stimulatory to the activity of the enzyme. EDTA and Hg++ were inhibitory.