Summary.The concentration of chlorpropham required to reduce primary root clongation by 50% was determined for more than 180 species of dicotyledons in laboratory tests and was found to vary from 0.01 ppm to more than 10 ppm. With certain exceptions, species within a family responded similarly; the Polygonaccae and Caryophyllaceae were the most susceptible and the Compositae were the most tolerant. There was a general correspondence between the test results and the relative susceptibility of species in the field. Tests with different varieties ofBrassies oleraceaandDaucus carotashowed no significant differences in varietal response.Tolérance comparée de quelques dicotylédones au chlorprop