A novel tyrosinase biosensor has been developed for a subpicomolar detection of phenols, which is based on the immobilization of tyrosinase in a positively charged chitosan film on a glassy carbon electrode. It was found that chitosan cross-linked with (3-aminooryloxypropyl) dimethoxymethylsilane is beneficial for the immobilization of tyrosinase. The large microscopic surface area and porous morphology of chitosan matrix lead to high enzyme loading, and the enzyme entrapped in this matrix can retain its bioactivity and the positively charged surface of chitosan can also display a good anti-interference ability to the substances with positive charge. Hence, the resulting sensor offers a high-sensitivity (150 nA.nM(-1)) for the monitoring of phenols, and the detection limit is as low as 5.0 x 10(-11) M. Its response time is less than 2 s reaching 95% of the steady-state value. It may retain 75% of the activity for at least 70 days.