Two different procedures have been used for the immobilization of enzymes on FET transducer for detecting pesticides. In the first, enzymes were entrapped in a photocross-linkable PVArSbQ membrane, while in the second, enzymes were cross-linked with BSA–glutaraldehyde membrane. Although the both membranes present the same Ž y6 detection limit for trichlorfon 10 M , PVA . rSbQ membrane shows a better reproducibility and above all, a longer lifetime.