The pectinesterase (pectin pectyl-hydrolase, EC 3.1.1.11) of the pulp of the banana fruit is completely solubilized by buffers containing 0.5M sodium acetate, and the activity of the enzyme in the pulp remains constant during ripening. An apparent change in the ease of extraction as ripening progresses may be artefactual. The enzyme has been purified 200-fold from climacteric fruit. In either case, a molecular weight about 30 000 is indicated by chromatography through Sephadex G200. After isoelectric focusing, the enzyme was found in two peaks, at pH 8.8 - 8.9 and at pH 9.3 - 9.4. The enzyme in the two peaks is of the same molecular size, and shows only minor differences in responses to pH, salt and polyol concentration during assay. These results are discussed in relation to previous publications concerning changes in pectinesterase activity and properties during ripening of banana fruits.