Abstract: For determination of the cited groups of insecticides, substrate solution (0.5 mM acetylcholine in 2.5 mM HEPES buffer solution of pH 8.0 containing 20 mM MgCl2, 100 mM NaCl and 0.01% of gelatin; 250 µL) was injected into a carrier stream (0.45 mL min-1) of HEPES buffer solution and the mixture was passed through a 10 cm single bead string reactor containing acetylcholinesterase (I) immobilized on glass beads (0.5 to 0.75 mm). The H+ produced was detected by a pH electrode with a wall-jet entry to assay I activity. Insecticide sample solution was passed through the reactor for 15 min instead of the working buffer. The working buffer was then reintroduced into the carrier line and the substrate solution was injected again to determine I activity. The concentration. of insecticide was determined by the inhibition of enzyme activity. I was reactivated by passing 20 µM 2-pyridine aldoxime methiodide solution through the reactor for 15 to 20 min. The method was applied in the analysis of simulated seawater. Calibration graphs are presented for paraoxon and malathion. Detection limits ranged from 0.5 ppb for malathion to 275 ppb for bromophos-methyl. A flow injection system, incorporating an acetylcholinesterase (AChE) single bead string reactor (SBSR), for the determination of some organophosphorus (azinphos-Et, azinphos-Me, bromophos-Me, dichlorovos, fenitrothion, malathion, paraoxon, parathion-Et, and parathion-Me) and carbamate insecticides (carbofuran and carbaryl) is presented. The detector is a simple pH electrode with a wall-jet entry. Variations in enzyme activity due to inhibition are measured from pH changes when the substrate (acetylcholine) is injected before and after the passage of the solution containing the insecticide. The percentage inhibition of enzyme activity is correlated to the insecticide concentration. Several parameters influencing the performance of the system are studied and discussed. The detection limits of the insecticides ranged from 0.5 to 275 ppb. The determination of these compounds was conducted in Hepes buffer and a synthetic seawater preparation The enzyme reactor can be regenerated after inhibition with a dilute solution of 2-PAM and be reused for analysis. The immobilized enzyme did not lose any activity up to 12 weeks when stored at 4°C.