A fiber-optic biosensor for the detection and determination of the pesticides carbaryl and dichlorvos was developed. The sensing bioactive material was a three-layer sandwich. The enzyme cholinesterase was immobilized on the outer layer, consisting of hydrophilic modified polyvinylidenefluoride membrane. The membrane was in contact with an intermediate sol-gel layer that incorporated bromcresol purple, deposited on an inner glass disk. The sensor operated in a static mode at room temperature and the rate of the inhibited reaction served as an analytical signal. Calibration curves were obtained for carbaryl and dichlorvos, with useful concentration ranges 0.11-8.0 mg l(-1) for carbaryl and 5.0-30 microg l(-1) for dichlorvos. The respective detection limits were 108 microg l(-1) and 5.2 microg l(-1). The method reproducibility was in the order of +/-3-5%. The method was successfully applied to the detection and determination of these pesticides in real water samples, without sample preparation steps. Recovery experiments were made and the accuracy of the method was 94.9%. No enzyme regeneration steps were applied and the sensor lifetime was 3 weeks (30% activity reduction). The bioactive mini sandwich can easily be replaced by simply unscrewing the terminal holding ring of the probe and placing a new sandwich, before the sensor is ready for use.