The stiffening of rubber‐like materials at low temperature involves several different phenomena, sometimes with their effects superimposed. One of these is crystallization. This is a rate process which is generally very fast at high stresses and very slow at zero stress. In these experiments at temperatures near −25°C and under a shear stress of about 148 p.s.i. the dynamic modulus of the rubber increased at a rate convenient to study. Correlation with x‐ray data showed that crystallization was very likely responsible for the increase in stiffness. The rate of change of stiffness increased rapidly with increase in applied stress, and there was no optimum rate at −25°C as has been found for unstressed rubber. The degree of vulcanization influenced the rate of change, tighter cures giving smaller changes. Neoprene FR, GR‐S and polybutadiene, which ordinarily show little evidence of crystallization showed very definite, but small increases in stiffness. Mixing GR‐S with natural rubber seems to limit the crystallization of the natural rubber rather effectively, but apparently Neoprene FR does not mix intimately enough with natural rubber to affect the crystallization of the latter appreciably.