The solvent-induced chiroptical properties of an optically active, regioregular (head-to-tail) polythiophene bearing chiral oxazoline residues, poly [3-{4-((R)-4-ethyl-2-oxazolin-2-yl)phenyl}thiophene] (poly-la), were investigated in mixtures of a good solvent, chloroform, and a variety of poor solvents by means of UV-visible and CD spectroscopies. Poly-la exhibited an induced circular dichroism (ICD) in chloroform upon the addition of poor solvents such as methanol and acetone with similar Cotton effect patterns, while some poor solvents such as acetonitrile and nitromethane dramatically changed the Cotton effect patterns of poly-la. The changes in the conformation, morphology, and the size of the poly-la aggregates induced by different solvents were also investigated with H-1 NMR titrations, X-ray diffraction, filtration experiments using membrane filters, and atomic force microscopy (AFM) measurements. On the basis of these results, possible models for the chiral supramolecular aggregates are proposed.