Phytotropins are a group of chemicals which have the ability to abolish the gravitropic response in plants by inhibiting the polar movement of auxin in the plant. They have other physiological properties such as inhibiting the phototropic response of stems. They bind to the naphthylphthalamic acid receptor and may elicit their physiological responses by this means. Most phytotropins consist of a benzoic acid moiety substituted at the ortho position by a bridging group connected to a second aryl group. Conformational energy calculations were performed on a subset of phytotropins. The calculations yielded a single, low energy conformation common to each molecule and thus identified three dimensional requirements for binding to the receptor. Electrostatic potential calculations, in the vicinity of the benzoic acid moiety, identified recognition and binding requirements for this group. Similar calculations for the second aryl group indicated that some similarity exists between the electrostatic potentials of molecules which bind most tightly to the receptor. The revised binding model was assessed by consideration of a second series of molecules showing phytotropic activity. The model was consistent with the biological activities of these molecules.