Chitosan was linked to cellulase (EC 3.2.1.4) from Trichoderma viride by covalent conjugation to periodate-activated carbohydrate moieties of the enzyme. The modified enzyme contained about 1.5 mol of polymer per mol of protein. The specific activity of the conjugate prepared was 39.8% of the native cellulase. The optimum pH and temperature for cellulase remained unaltered after modification. The thermostability was increased bg 8.9 degreesC for the cellulase-chitosan complex. Thermal inactivation at different temperatures ranging from 65 degreesC to 80 degreesC was markedly increased for the polymer-modified enzyme. The stability within the pH range 1.0-3.2 was also improved for the modified enzyme.