Elongational flow-induced crystallization in supercooled liquid poly(ethylene terephthalate) (PET) was investigated via elongational flow opto-rheometry (e.f.o.r.) and temperature-modulated differential scanning calorimetry (t.ms.c.) in the temperature range 100-130 degrees C, in which the blow moulding operation on PET is usually conducted. Measurements of tensile stress sigma(t) and birefringence Delta n(t) as a function of the Hencky strain rate <(epsilon)over circle>(0) and time t with e.f.o.r. revealed the features of their time evolution as being due to spherulite and/or flow-induced oriented crystalline formation during the elongation process. Below 110 degrees C, when spherulite growth was negligible, elongational viscosity eta(E)(t) ( = sigma(t)/(E) over circle(0)) determined at different <(epsilon)over circle>(0) first developed slowly and exhibited ＇up-rising＇ or ＇hardening＇ after certain periods of time t(eta E); this behaviour reduced to a characteristic Hencky strain epsilon(eta E) ( = <(epsilon)over circle>(0)t(eta E)) congruent to 1.0, independent of <(epsilon)over circle>(0), reflecting the formation of flow-induced oriented crystallites beyond epsilon(eta E). At higher temperatures (similar to 120-130 degrees C), however, where spherulites that contributed significantly to eta(E) but little to Delta n(t) were formed during the resting period and/or in the early stage of elongation, the critical up-rising strain epsilon(eta E) varied from epsilon(eta E) congruent to 0.5 to epsilon(eta E) congruent to 1.0 as <(epsilon)over circle>(0) was increased. Eventually the spherulites appeared to be deformed or broken down to form oriented crystallites. Analyses via t.ms.c. combined with e.f.o.r. identified spherulites in the specimens elongated at high temperatures but flow-induced oriented crystallites in those elongated at low temperatures.