Summary.Picloram was incubated at concentrations of 0, 0·25, 0·5 and 1 ppmw in Regina heavy clay topsoil maintained at moisture‐holding capacity and a t 25° C The herbicide was quantitatively bioassayed at regular intervals. There was a lag period prior to degradation at all concentrations. The duration of the lag period increased as the concentration of picloram in the soil was increased. Al the end of each lag period, the degradation of picloram occurred as a first‐order reaction, i.e. the rate of degradation was independent of the concentration of picloram in the soil. Due to the pronounced variations in the duration of the lag periods with concentration, the assignment of half‐value or half‐life periods to picloram degradation in soil is considered to be of doubtful significance.Etude de la degradation de t'acide 4‐amino‐3,5,6‐trichloropicol