7‐Chloroindoleacetic acid and dichloroindoleacetic acids with a Cl in the 7 position showed anti‐auxinic activity and promoted root growth in wheat (Triticum aestivumL. cv. Diamant II). In contrast, 4‐, 5‐ and 6‐chloroindoleatetic acids acted as strong auxins inhibiting the growth of wheat roots. Flax (Linum usitatissimumL. cv. Concurrent) and cucumber (Cucumis sativusL. cv. Favör) roots showed similar, but less clear‐cut responses. 7‐Chloroindoleacetic acid and 4,7‐dichloroindoleacetic acid alleviated root growth inhibition in wheat caused by IAA, monochloroindoleacetic acids and benzyladenine. 2,4‐D, 4‐ and 6‐chloroindoleacetic acids strongly induced ethylene formation in cucumber seedlings; 4,7‐ and 6,7‐dichloroindoleacetic acids did not, except at high concentrations. The more lipid‐soluble dichloroindoleacetic acids were stronger inhibitors of ATP formation in cucumber mitochondria than monochloroindoleacetic acids, while IAA itself h