Elongational flow-induced crystallization behavior was investigated on supercooled liquids of two poly(ethylene naphthalate)s (PENs) with different crystallization habits via elongational flow optorheometry (EFOR), temperature-modulated differential scanning calorimetry (TMDSC), and Rayleigh scattering in the temperature range 170-190 degrees C. The samples were an antimony-catalyzed PEN (Sb-PEN) with a rapid crystallization rate but less well-organized spherulites and a germanium-catalyzed PEN (Ge-PEN) with a temperature sensitive (slow) crystallization rate but better organized spherulites. In the elongation of Sb-PEN at 170 degrees C, a strong tendency of strain hardening was observed in the early stage and transformation of the spherulites into rodlike morphology occurred in the later stage, accompanying a substantial increase in birefringence. On the other hand, for Ge-PEN at 170 degrees C, molecular orientation along the flow direction proceeded and flow-induced crystallization took place rather suddenly at a Hencky strain of epsilon approximate to 2.65, with the crystalline lamellae irregularly growing transverse to the oriented chains. However, in Ge-PEN elongated at 190 degrees C, especially with a low strain rate epsilon(0) (<0.01 s(-1)), stable spherulites were formed and their growth dominated the subsequent elongation behavior. In these PENs, the features of the flow-induced structure development were governed by the dimensionless strain rate, which was the ratio of epsilon(0) to the spherulite growth rate under the quiescent state. Depending on the dimensionless strain rate being above or below the critical value, either the oriented-crystallite formation or the spherulite growth dominated the elongation behavior, respectively.