The physiological activity of complete series of mono‐ and di‐substituted chloro‐and methyl‐phenoxyacetic and benzoic acids have been investigated using the wheat cylinder, pea segment and pea curvature tests. In the phenoxyacetic acids, high activity was associated with substitution in the 3‐, 4‐, 2:4‐, 2:5‐ and 3:4‐positions and in general, chlorine had a greater effect than methyl in conferring activity.With the benzoic acids, 2:3‐, 2:5‐ and 2:6‐, disubstitution gave active compounds, the chloro‐derivatives again being the more active. The 2:5‐compound was the most active in the dichloro‐ series, all members of which were less active than the 2:3:6‐trichloro‐ and 2:3:5:6‐tetrachloro‐acids. Benzoic acids substituted in the 4‐ position were either inactive or exhibited only weak growth‐promoting activity.The results are discussed in relation to recent theories which attempt to relate growth‐promoting activity with the po