A flow-injection spectrophotometric procedure was developed for screening organophosphorus pesticides. The method is based on the inhibition of acetylcholinesterase immobilized on controlled porous glass beads with acetylcholine chloride as the substrate. Methyl parathion, chlorpyrifos, malathion and dichlorvos have been tested. The analytical peak height for a given acetylcholine chloride concentration correlates linearly with the logarithmic concentration of the pesticides between 1.0 × 10-3 mol L-1 and 1.0 × 10-6 mol L-1. If bromine water is added to the pesticide solution, a dramatic increase is observed in the analytical signal, and correlations with the logarithm of the concentrations are observed from 1.0 × 10-3 to 1.0 × 10-10 mol L-1.