A new 4-ethynyl-substituted monomer, bis(4-ethynyl phenyl) 1,4,5,8-naphthalimide (EPNI) was synthesized and characterized. EPNI is a crystalline material which, when thermally polymerized under N2 in the solid state, yielded a highly crosslinked resin. The progress of polymerization was followed by Fourier transform i.r. spectroscopy, differential scanning calorimetry and wide-angle X-ray diffraction. It was found that EPNI reacted completely thermally to yield an extended polyene structure. Polymer plates were made by solid-state polymerization of EPNI under 4.8 MPa for 1.5 h at 300-degrees-C. Because EPNI is ordered and polymerized in the crystalline state, it showed zero polymerization shrinkage (EPNI had the same density as the well consolidated polymer, 1.40 g ml-1). EPNI polymerized to high conversion in spite of its very rigid matrix, probably because of the localization of its polymerizable ethynyl groups. A plate of polymeric EPNI (PEPNI) with 10% voids had a storage modulus of 2.2 GPa at room temperature, a glass transition temperature, T(g), of 309-degrees-C and a linear coefficient of thermal expansion of 4.28 x 10(-5) K-1. PEPNI has excellent thermal and thermooxidative stabilities since it retained over 83% of its initial weight after 105 h at 427-degrees-C in air.