As part of molecular recognition studies on the phytohormone indole-3-acetic acid (IAA) a series of alkylated IAAs has been examined. Phenyl-ring substitution (alkyl = methyl and ethyl) at positions 4-, 6- or 7- as well as pyrrole substitution at the 2-site resulted in the six compounds which are analyzed: 2-Me-IAA, 4-Me-IAA, 6-Me-IAA, 7-Me-IAA, 4-Et-IAA and 6-Et-IAA. The structure-activity relationships investigated include those between the geometrical parameters of the molecular structures determined by X-ray analysis, the growth-promoting activities in the Avena coleoptile straight-growth bioassay and relative lipophilicities calculated from retention times on a reversed-phase HPLC column and from R(F) values in reversed-phase TLC. Lipophilicities are correlated with the moments of inertia, average polarizability, molecular mass, and the van der Waals radii of the ring substituents. The influence of substitution on the electronic properties of the indole ring and its geometry is discussed on the basis of the UV and 1H NMR spectra.