The circular dichroism (CD) of a polythiophene in the molecularly dispersed state was observed for the first time. The polythiophene possesses two chiral alkyl side chains (poly{3,4-bis[(S)-2-methyloctyl]thiophene}, PDMOT). The chiroptical properties of this polymer in solution, aggregate, and thin solid film states were investigated. A chirally distorted main-chain conformation in a single molecule of PDMOT accounts for the observed CD spectra in solution and could also be considered as a novel chiral motif in an optically active polymer, besides the highly ordered, intramolecular helical structure and the chiral superstructure resulting from intermolecular pi-pi stacking. Significant temperature and solvent effects on the CD were also observed in the spectra.