Greenhouse experiments were conducted with 35 plant species to evaluate the selectivity and symptoms caused by quinclorac (BAS 514 H; 3,7-dichloro-8-quinolinecarboxylic acid) as compared with 2,4-D (2,4dichlorophenoxyacetic acid). The grass species tested fell into groups either susceptible or tolerant to quinclorac based on their sensitivity and morphological responses. The susceptible group included Echinochloa spp., Setaria spp., Digitaria sanguinalis, Panicum dichotomiflorum and Zea mays, and the tolerant group included Oryza sativa, T riticum aestivum, Sorghum bicolor, Avena sativa, Elusine indica, Dactylis glomerata, Arthraxon hispidus and Alopecurus aeqularis. Quinclorac-susceptible grasses showed different symptoms from those caused by 2,4-D: chlorosis, necosis of newly emerged leaf, and finally stunting and desiccation of the entire plant. Most broadleaves were susceptible to quinclorac and showed auxin-like symptoms, except for Brassica napus, Raphanus sativa, Capsicum annuum and Commelina communis, which were tolerant. The auxin activity of quinclorac was evaluated by bioassays. In mesocotyl elongation response of barnyardgrass and sorghum, 2,4-D showed the auxin activity on both species, but quinclorac showed it on sorghum alone. In petiole curvature response of radish and velvetleaf, 2,4-D showed the auxin activity on both species, quinclorac showed it only on velvetleaf. It is suggested that the mode of action of quinclorac on susceptible grasses is not related to auxin activity.