1-Chloro-2-beta-naphthylacetylene (Cl beta NA) polymerized in good yields in the presence of MoCl5-based catalysts. The highest weight-average molecular weight of poly(Cl beta NA) reached about 3 x 10(5). The polymer was a yellow solid (absorption cutoff in CHCl3 450 nm). It was soluble in toluene, chloroform, etc., and provided a tough film by the solvent casting method. The polymer retained its weight up to 300 degrees C in air; it was thermally less stable than poly(1-chloro-2-phenylacetylene) but more stable than poly(beta-naphthylacetylene). The oxygen permeability coefficient (P-O2) of this polymer was 19 barrers (25 degrees C), which is fairly small for a substituted polyacetylene.