The effect of long-term application of pendimethalin in a maize±wheat rotation on herbicide persistence was investigated. Pendimethalin was applied at 1.5 kg AI ha À1 separately as one or two annual applications for ®ve consecutive years in the same plots. Residues of pendimethalin were determined by gas chromatography. Harvest-time residues of the herbicide decreased gradually over the years and at the end of ®ve years less than 3% of applied pendimethalin was recovered from soil as against 18% in the ®rst year. Residues were found distributed in the soil pro®le up to 90 cm depth at the end of the experiment with peak distribution of 0.03 mgg À1 in the surface layer of soil treated with 10 herbicide applications. The minimum distribution was, however, in the deepest soil (75±90 cm) pro®le. Some of the metabolites of pendimethalin ie dealkylated pendimethalin derivative, partially reduced derivative and cyclized product were also traced in surface and sub-surface soils up to 90 cm. A study of the rate of degradation of pendimethalin in ®eld-treated soils under laboratory conditions revealed faster degradation compared to control soils. Only the surface soil (0±15 cm) showed this enhanced degradation of the herbicide, which could be due to the adaptability of the aerobic micro-organisms to degrade pendimethalin. Microbes capable of degrading herbicide were isolated, identi®ed and pendimethalin degradation was con®rmed in nutrient broth.