<p>A heat-resisting optical fiber comprising a core and a cladding. The core is made of a polymer comprising units of a methacrylic ester, of which the ester moiety has an alicyclic hydrocarbon group of not less than 8 carbon atoms. The cladding is made of a transparent polymeric material having a refractive index at least 3% smaller than that of the core. This optical fiber has excellent heat resistance and good flexibility and can be used at a high temperature over 110.degree.C.</p>
