Abstract: The FIA manifold for the determination of metriphonate (trichlorfon), (I) consisted of two injection valves, an enzyme column and a spectrophotometric detector connected in series. At the first injection valve, 100 µL sample was injected into the 0.1 M phosphate buffer carrier stream of pH 8.5. When the sample zone reached the second injection valve, 70 µL 2 mM acetylthiocholine iodide was added. The mixture was propelled to the enzyme column (2.5 mm x 2.5 mm i.d.) containing acetylcholinesterase (AChE) immobilized onto controlled porosity glass beads. The flow was stopped for 1 min to allow the AChE-catalyzed hydrolysis of acetylthiocholine to proceed. The flow was started at the end of the reaction period and the reaction product (thiocholine) was swept towards the spectrophotometric detector. Before reaching the detector the flow was merged with 1 mM 2,2'-dithiobis(2-nitrobenzoic acid) to enable detection at 405 nm. The presence of I inhibited the hydrolysis of acetylthiocholine and reduced the absorption signal. A linear calibration graph was obtained for 1-10 µM-I, the detection limit was 0.8 µM and the RSD (n = 5) for 6 µM-I was 2%. The sample throughput was 30 samples/h.