AbstractIn laboratory tests with 115 species of dicotyledons, the concentration of pronamide in water for 50% inhibition of root elongation ranged from 0.03 to >8 parts/million. With certain exceptions, species within a family responded similarly. Polygonaceae and Caryophyllaceae were the most susceptible and Compositae the most tolerant, as they were also to chlorpropham. There was good correspondence between the test results and the relative susceptibility of species to pre‐emergence application in the field.