Elongational flow-induced crystallization behaviour was investigated on supercooled liquids of two poly(ethylene terephthalate)s (PET) with different crystallization habit via elongational how opto-rheometry (EFOR), temperature-modulated differential scanning calorimetry (TMd.s.c.) and Rayleigh scattering in the temperature range of 100-130 degrees C usually employed for blow moulding operations. The samples were an antimony catalysed PET (Sb-PET) with rapid crystallization rate but less-ordered spherulites, and a germanium catalysed PET (Ge-PET) with temperature sensitive (slow) crystallization rate but better organized spherulites. In the elongation of Sb-PET at 110 degrees C, transformation of the spherulites into rod-like morphology occurred even in the early stage, which was followed by a strong strain hardening tendency, accompanying the increasing of birefringence. On the other hand, for Ge-PET at 110 degrees C, molecular orientation along the flow direction preceded and then flow-induced crystallization with crystalline lamellae growing transverse to the oriented chains took place rather suddenly at a Hencky strain epsilon of approximate to 2. However, in Ge-PET elongated at 130 degrees C especially with low strain rate (epsilon)over dot(0), spherulite growth dominated the rheology. In these PETs, the features of the flow-induced structure development were governed by the dimensionless strain rate that was the ratio of Bo to the spherulite growth rate under quiescent state. Depending on the dimensionless rate being above or below a certain critical value, the oriented crystallite formation or the spherulite growth dominated the behaviour, respectively.