Poly(2-ethynylthiophene) was stereoregularly prepared with a Rh complex catalyst, [Rh-(norbornadiene)Cl](2), to selectively produce the cis-transoid isomer in high yields when triethylamine was used as the polymerization solvent at 20 degreesC. Photoinduced cis-to-trans isomerization was newly found to take place when the film of a pristine cis-transoid polymer was photoilluminated using a Xe lamp with light of wavelength, 320-500 nm under vacuum for 5 h. The cis and trans polymers obtained before and after the illumination were characterized in detail using H-1 NMR, UV-vis of solution and film, and electron spin resonance methods.