Strain-induced crystallization (SIC) is studied in blends containing 92 wt% ofcis-l, 4-polyisoprene (hevea rubber) and 8% of synthetictrans-l, 4-polyisoprene (t-PI), for different molecular weights of t-PI. The initial crystallinity (in the unstrained state) is analyzed in terms of the effect of both molecular weight and the microgel content. The SIC of the natural rubber was measured using the wide-angle x-ray scattering technique. Uncrosslinked solution blends were uniaxially stretched at room temperature, and the dependence of the induced crystallinity on elongation and molecular weight of the t-PI component was analyzed. Also, the molecular weight dependence of the induced crystallization rate is discussed.