A filament stretching rheometer is used to investigate the extensional flow-induced crystallization of two commercial grade isotactic poly-1-butene samples. The degree of crystallinity of the stretched fibers is quantified using differential scanning calorimetry measurements as a function of extension rate and accumulated Hencky strains. All the measurements are performed using the Janeschitz-Kriegel protocol. The samples are first melted to erase their thermal and mechanical history. They are then quickly quenched to T=98°C after which the stretch is imposed. The deformed filament is then allowed to crystallize fully at T=98°C. The extensional rheology of both the samples shows only minimal strain hardening. For the case of the lower molecular weight sample, the percent crystallinity increases from 46% under quiescent conditions to a maximum of 63% at an extension rate of ε̇ =0.05 s-1. This corresponds to an increase of nearly 50% above the quiescent case. The high molecular weight sample shows similar trends achieving an increase in crystallinity of 25%. The experiments show an optimal extension rate for which the extensional flow has the maximum impact on the polymer crystallinity. The percent crystallinity of both the samples is observed to increase with increasing strain for a fixed extension rate. Small angle X-ray scattering shows that the observed increase in crystallinity is likely due to the increasing orientation and alignment of the polymer chains in extensional flows which enhances the thread-like precursors responsible for the formation of the crystals in the shish-kebab morphology. © 2011 The Society of Rheology, Inc.