SUMMARYThe plant growth‐regulating activities of chloro‐, bromo‐ and iodo‐isomers in 2,5‐, 2,3,5‐ and 2,3,6‐substituted benzoic acids were assessed in the wheat cylinder, pea segment and pea curvature tests. Their effects in the tomato‐leaf epinasty test were also investigated.Replacing anortho‐chloro atom by bromine had little or no effect on activity. Anortho‐iodo substituent, however, reduced activity in the 2,3,6‐substituted series and almost completely inhibited it in the 2,5‐ substituted series. This decreased activity ofortho‐iodo‐substituted isomers is visualized as being due to a steric inhibition of the attachment of the carboxyl group to its receptor unit prior to the initia