The beta phase of poly(ethylene naphthoate) is suggested by high resolution electron microscopy and electron diffraction data to be monoclinic, with a = 1.320 nm, b = 0.962 nm, c = 1.300 nm, alpha = 131.50 degrees, beta = 90 degrees, and gamma = 90 degrees and space group P2(1)/n 1 1. A model structure has been presented which accounts quantitatively for the observed intensities of the hk0 reflections and provides qualitative agreement with the hk1 data.