SUMMARYThe plant growth‐regulating activities of all the mono‐ and di‐chloro‐substituted α‐hydroxy‐phenylacetic (mandelic) acids and their methoxy derivatives have been determined in the wheat coleoptile cylinder, pea segment and pea curvature tests. In general, chloro‐substituted α‐hydroxy acids were less active in all three tests than the corresponding α‐methoxy acids. The α‐methoxy compounds and the dichloromandelic acids were more active in pea than in wheat tissues.These results are discussed in relation to the plant growth‐regulating activities of the corresponding phenylaceti