SummaryIn an investigation of the possible reasons for tolerance lo fluroxypyr byGalium aparineL. (cleavers), a wide range of variation in total herbicide retention was exhibited by nine diverse populations from throughout Europe. Although these differences in fluroxypyr relention became negligible when considered on a unit area basis, a two‐fold difference was maintained per unit dry matter. There was no difference in fluroxypyr uptake between a fluroxypyr‐tolerant and a fluroxypyr‐susceptible population, and differences in translocation between the two populations did not seem to explain the differential tolerance. It is suggested that differential metabolic detoxification may be the main reason for the variation in response to fluroxypyr by populations ofG. ap