The polymerization of β-naphthylacetylene by WCl- and MoCl-based catalysts was investigated. The WCl and MoCl catalysts were both found to be highly active. The polymer obtained with WCl [poly(βNA)–W] was soluble, and its highest molecular weight was 9, 200, which is by far the highest value reported so far. The polymer formed with MoCl [poly(βNA)–Mo] was hardly soluble. According to the results from IR spectroscopy and differential thermal analysis, poly(βNA)–W was rich in trans structure, whereas poly(βNA)–Mo was cis-rich. The X-ray diffraction patterns showed that poly(βNA)–W was amorphous, while poly(βNA)–Mo was crystalline. The heat treatment of poly(βNA)–Mo caused cis-to-trans isomerization, and rendered the polymer more soluble. The electric conductivity and paramagnetic property of the polymers are also described. © 1979 The Society of Polymer Science, Japan.