Novel, optically active, cis-transoidal poly(phenylacetylenes), poly((R)-(-)- or (S)-(+)-(4-((l-(l-naphthyl)ethyl)carbamoyl)phenyl)acetylene) (poly(R)-1 and poly(S)-1), were prepared, and their chiroptical properties were investigated by means of circular dichroism spectroscopy. We have found that the poly-1's have a predominant one-handed helical conformation in solution, and the helix-sense undergoes a transition from one helix to another in response to external, chiral, and achiral stimuli, such as temperature, solvent, and chiral interaction with small molecules.