Application of electron diffraction (ED) to poly(ethylene naphthalate) (PEN) crystallized isothermally from the melt at 260°C has permitted determination of the α-and β-form unit cells. While the α-form lattice parameters and cell agree well with those proposed in the literature, a different cell is proposed for the β-form (monoclinic, P21/n11 space group; 4 chains, with a = 1.304 nm, b = 0.926 nm, c = 1.300 nm, α = 131.47°, β = γ = 90°, ρ = 1.368 g/cm). The β-form crystals were large, resembling hedrites (axialites). A number of different, single-zone ED patterns from at least one additional two-chain unit cell (γ) were found, indicating a large degree of PEN polymorphism. The γ unit cell has not yet been determined. Nascent melt-polymerized samples were also examined, giving rise to both β-and γ-type patterns. The γ-patterns arise from extended-chain single crystals of variable thickness.