Copolymerizations of phenylacetylene with various polycyclic olefins (norbornene, dimethanooctahydronaphthalene, deltacyclene, norbornadiene, benzonorbornadiene, ethylidenebicycloheptene and trimethylsilylnorbornadiene) by WCl6-n-Bu4Sn catalyst were investigated. Phenylacetylene and these cycloolefins were simultaneously consumed. Formation of random copolymers was confirmed by the g.p.c. curves, u.v.-visible spectra and H-H COSY n.m.r. spectra of the copolymerization products. The copolymer composition curves were obtained and the monomer reactivity ratios were determined. The reactivities of these cycloolefins were close to or lower than that of phenylacetylene. Most of the r(1) x r(2) values were about unity or lower. In the case of norbornene-type monomers, the larger the ring strains of cycloolefins, the larger the reactivities to the phenylacetylene propagating end. Monocyclic olefins were scarcely or not consumed in the copolymerization with phenylacetylene.