A combination of tensile stress, rheooptical birefringence, and wide-angle X-ray scattering (WAXS) Was used to probe the dynamic response of the low-tacticity ether-soluble (ES) fraction of elastomeric polypropylene (ePP) derived from metallocene 2-arylindene hafnium. catalyst. The ES fraction has isotactic pentad distribution [mmmm] = 21% and a very low amount of crystallinity (less than or equal to2% by differential scanning calorimetry and WAXS). In tensile stretching and step-strain shearing, ES exhibits unusual deformation behavior of crystalline chains preferentially oriented orthogonal relative to the deformation axis. Under deformation, WAXS; shows arcing along the meridian axis at a scattering angle 2theta = 16.0degrees (d = 0.551 +/- 0.002 nm) which coincides with one of the characteristic reflections of the beta-form, but the higher order reflection for the beta-form at 2theta = 21.3degrees is not observed. The meridional arcing, which signifies crystallization of the low-tacticity fraction of ePP, is also observed when ES is blended with higher-tacticity fractions of ePP. The meridional arcing, however, is observed at 2theta = 14.0degrees, corresponding to (110) reflection of the alpha-form, instead of at 2theta = 16.0degrees for the neat ES. The crystallization in the alpha-form offers evidence of cocrystallization of the ES fraction with the higher-tacticity components in the same crystalline form as the host matrix. We believe that the cocrystallization occurs through an epitaxial growth in the ac-faces of the alpha-form.