A sensitive and selective phosphorimetric method for the determination of 1-naphthaleneacetic acid (1-NAA) based on a flow-injection system connected to a flow cell packed with a solid support and placed in the sample compartment of a conventional luminescence spectrometer is described. A non-ionic solid polymeric resin Amberlite XAD-7 is used for the packing. After injection of the sample, 1-NAA is on-line retained in the packed resin and measurements of the heavy atom induced (HAI)-room temperature phosphorescence (RTP) emission (lambda(ex)/lambda(em)=292/490nm) from this native luminescent compound are taken. The optimum experimental conditions were investigated by injecting 2ml samples of an aqueous solution of 1-NAA in the flow system. A concentration 0.15moll(-1) of thallium(I) ions, as heavy atom, both in the samples and the carrier flow, was finally selected. Also, a concentration of 6mmoll(-1) of sulphite was optimal for ensuring the necessary deoxygenation of the system at the selected flow rate of 0.8mlmin(-1). After measurement, the solid support was efficiently regenerated by injecting 1ml of a mixture water:acetone in a ratio 1:1 (v/v) into the flow. The detection limit (3sigma criterion) was 1.2ngml(-1) of 1-NAA. The repeatability (R.S.D.) for five replicates of a sample containing 50ngml(-1) of analyte turned out to be +/-3% and the calibration graphs proved to be linear up to 500ngml(-1) of 1-NAA (maximum concentration assayed). The effect of potential interferences from other organic species which can be also used as plant growth regulators, as well as from various inorganic cations and anions, has been investigated as well. The method was successfully applied to the determination of low levels of this plant growth regulator in natural waters (river and fountain waters) and apples.