Fifteen series of aryloxyacetic, α‐aryloxy‐propionic, ‐butyric and ‐isobutyric acids and the corresponding arylthio‐analogues have been synthesized and their growth‐regulating activity has been assessed in the tomato‐leaf epinasty, wheat cylinder and the pea curvature tests. It is shown that in general arylthio‐derivatives are less active than their aryloxy‐analogues. Although for a cell elongation response the presence in the molecule of an a‐hydrogen atom is usually necessary, three exceptions to this generalization have now been found. A lower activity in the 3:5‐dichloro‐ compared with the 2:6‐dichloro‐compounds has been demonstrated. Evidence is presented to show that an ortho hydrogen atom in the nucleus is not an essential requirement for cell elongating activity as is