After an 88-fold purification, pectinesterase produced by a strain of Aspergillus niger, isolated from rotten lemons, showed the following main characteristics: maximum activity at 45 degrees C, pH 5; Km, with pectin as substrate, 1.01 mg/L; Delta G(*), 4750 Cal/mol. Polygalacturonic acid and methanol acted as competitive and non-competitive inhibitors, respectively. The activity of the enzyme was impaired by MgCl2 and stimulated by NaCl.