Flow injection analysis(FIA) was examined for the fluorometric determination of ammonia in river water. Ammonia reacted with ο-phthalaldehyde(OPA) in the presence of 2-mercaptoethanol(ME) to form a fluorescent substance at pH 9.5. The reagent solution containing 10^-2 M OPA and 10^-3 M ME and the carrier fluid(distilled water) were propelled by a double plunger pump at a rate of 1.2 ml/min. The 40μl sample solution, injected into the carrier stream, was mixed with the reagent solution in a Teflon tubing (3 m, 0.5 mm i.d.) and led to a flow cell(18 μl). Fluorescence excited at λ_ex=350 nm was measured at λ_em=486 nm. Ions present commonly in river waters did not interfere with the determination of ammonia. An anion exchange column installed just behind a sample injection valve in the flow system was effective in eliminating interferences with amino acids. Using the proposed FIA system, trace amounts of ammonia(3150 ppb as nitrogen) in river water were determined in the rate of 40 samples per h.