A differential pulse polarographic method for the determination of the herbicide thiazopyr has been developed. The polarographic study of thiazopyr exhibited two well-defined cathodic peaks within the pH range of 1.0 to 8.0. The variation of pH and polarographic parameters indicated that the optimum conditions under which thiazopyr could be reduced were a pH 7.0 BR buffer solution, a reduction peak potential of -1270mV (vs. SCE), scan rate of 5mVs(-1), pulse amplitude of 50mV with pulse duration of 50 ms at an ambient temperature of 25 +/- 3 degrees C. The main reduction peak was characterised by cyclic voltammetry as being irreversible and diffusion-controlled. A linear relationship between the peak current and the concentration of thiazopyr was obtained in the range of 0.43-38.6 mu gmL(-1), with a detection limit of 0.127 mu gmL(-1). The proposed method was successfully applied to the determination of thiazopyr in spiked fruit juice and soil samples. The mean recoveries of the 19.8 mu g g(-1) and 3.96 mu gmL(-1) thiazopyr spiked to soil and orange juice were 20.2 +/- 1.0 mu gg(-1) and 3.84 +/- 0.12 mu gmL(-1), at 95% confidence level, respectively. The sufficiently good recoveries and low relative standard deviation (RSD) data confirm the high accuracy and precision of the proposed method. The interferences effects of several commonly used pesticides and inorganic species were also studied. Interfering effects were eliminated either by providing selectivity with pH, or using EDTA as complexing agent.