A method is described for the construction of a potentiometric catechol biosensor by entrapment of tyrosinase (polyphenol oxidase; PPO) into a conducting polypyrrole film on a platinum electrode. The optimum conditions for the formation of the PPy-Tyr (polypyrrole-tyrosinase) film include a current density of 0.5 mA cm-2, a polymerization period of 150 s, 0.1 M pyrrole and 50 U mL-1 tyrosinase. The presence of tyrosinase in the polymer film was confirmed by scanning electron microscopy (SEM), cyclic voltammetry (CV) and potentiometric measurements. The PPy-Tyr biosensor gave a sensitivity of 10 mV/μM for catechol, with a response time of 80 s. The optimum potentiometric measurement of catechol was achieved in 0.1 M phosphate buffer within a pH range of 6.5-7.5 and the lowest detectable concentration under these conditions was 1.0 μM. A linear concentration range of 1-16 μM catechol was obtained based on Nernstian response, but a non-Nernstian response demonstrated a linear range of 1-50 μM. The PPy-Tyr biosensor was stable for at least 1 month when stored in a buffer at about 4 °C.