Racemic 2-(5,7-dichloro-3-indolyl)propionic acid (5,7-Cl-2-2-IPA) was synthesized from 5,7-dichloroindole-3-acetic acid by successive esterification, methoxycarbonylation, methylation, and double hydrolysis. The racemate was converted to diastereomeric esters of l-menthol; these were separated by recycling HPLC into two optically active diastereomers; that were then hydrolyzed with p-TsOH to two optically active enantiomers of 5,7-Cl-2-2-IPA. The absolute configurations of both these enantiomers were determined by comparing the H-1-NMR spectra of their diastereomeric l-menthyl esters with those of the diastereomeric l-menthyl esters of 2-(3-indolyl)propionic acid (2-IPA) of known absolute configurations. An assay by the coleoptile elongation of Avena sativa showed the (S)-(+)-enantiomer of 5,7-Cl-2-IPA to have weak auxin activity, whereas the (R)-(-)-antipode had no auxin activity at any concentration tested. Interestingly, the (R)-(-)-enantiomer had antiauxin activity very close to that of 2-(5,7-dichloro-3-indolyl)isobutyric acid (5,7-Cl-2-IIBA), a strong antiauxin. These data indicate that, of the two methyl groups in its molecule, the antiauxin activity of 5,7-CL2-IIBA was due only to the (R)-methyl group.