Thiazopyr residues have been determined in soil, grass weeds and seedlings of woody plants by gas chromatography. The herbicide was extracted from soil and plant samples with ethyl acetate and plant extracts were cleaned-up on a Florisil column. Thiazopyr levels were determined by gas chromatography with nitrogen-phosphorus detection and confirmed by gas chromatography with mass spectrometry, ion trap detection. The average recoveries were always higher than 97% with a relative standard deviation between 1 and 6%. The limit of detection for the herbicide was, at least, 0.01 μg/g in soil and 0.02 μg/g in plant with both gas chromatographic methods. Samples of soil and Pinus pinea L. plants, treated with thiazopyr, were also analyzed.