Synthetic cis-1,4 polyisoprene prepared by a Ziegler catalyst and naturally occurring cis-1,4 polyisoprenes from Hevea brasiliensis and Lactarius volemus were crystallized at -80 to -45 degrees C from dilute hexane solution. The melting temperature, T-m, of these solution-grown crystals in hexane solution was determined by differential scanning calorimetry. The T-m value of natural rubber from Hevea brasiliensis was higher than that of low molecular weight rubber from Lactarius volemus and synthetic cis-1,4 polyisoprene, and was not varied by removing proteins, mixed fatty acids and linked fatty acids. For fractionated samples, the T-m value of rubber from Lactarius volemus decreased in proportion to the molecular weight, while that of natural rubber was independent of the molecular weight.