The microhardness technique has been used to follow the crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) from the glassy state. The crystallization temperatures investigated ranged from 423 to 448 R. Results are analyzed on the basis of the Avrami equation. Avrami parameters are discussed in the light of the morphology developed during the isothermal crystallization of PEN, as revealed by preceding electron microscopic studies [Balta Calleja et al. J Macromol Sci Phys 1998;37:411]. The temperature dependence of microhardness for amorphous PEN has been also investigated from room temperature up to similar to 30 K above the glass transition temperature, T-g, of the material. From these measurements, the T-g value has been derived in agreement with preceding calorimetric data [Buchner et al. Polymer 1989;30:480].