The environmental fate of mepiquat-chloride was investigated in a number of laboratory studies using [2,6-14C] mepiquat-chloride. Results of laboratory aerobic route and rate of degradation studies carried out at 10, 20 or 25°C and either 75% Field capacity at 1.3 bar or 40% MWHC were available for 6 soils. No major metabolites (>10% AR) were present in any soil at any time point. The only metabolite identified was 4-OH mepiquat-chloride, which peaked at 1.6% AR after 7 d in a loamy sand soil. At 121 d, 14CO2 represented a maximum of 69.7% AR, and at 120 d unextracted radioactivity accounted for a maximum of 43.7% AR. Laboratory aerobic first order DT50 values (20 to 25°C, 40% MWHC or 75% FC at 1/3 bar) for mepiquat-chloride were in the range 6 to 40 d (see Table B.8.13 for further details). The longest laboratory DT90 was 133 d at 20°C. At 10°C the aerobic degradation of mepiquat-chloride was significantly slower, with a DT50 value of 83 d at 40% MWHC. There did not appear to be any strong correlation between the mepiquat-chloride degradation rate and soil pH or organic carbon content.