Abstract The movement and dissipation of metolachlor [2-chloro-6′-ethyl- N-(2-methoxy-1-methylethyl) ace- o-toluidide] were evaluated in plain field wet tropical conditions in Martinique (French West Indies) and in a Mediterranean area (Languedoc, South of France). Adsorption parameters are discussed with the Freundlich equation, and desorption evaluated using a model with two compartments corresponding of two different energy levels. Higher clay content in tropical and ferrisol involved adsorption parameters ( K fa) higher than that found for the Mediterranean fluvisol, as also for K 1, K 2 and Q ads n desorption parameters. During our studies, rainfall was largerly more important in tropical conditions. As these two main factors (adsorption-desorption parameters and rainfall) partly conterbalanced each other, metolachlor aptitude to leaching appeared relatively moderate under the conditions chosen for this work. Half-life of the compound (in the 0- to 25-cm soil layer) varied from 21 to 31 days.