The semitheoretical expression,fc=&Dgr;n0−csF&Dgr;nc0−cs,wherefc=volume fraction crystalline, &Dgr;n0=observed birefringence,c=the stress‐optical coefficient,s=the stress on actual cross section,F=a known function of crystallite orientation, and &Dgr;nc0=the birefringence of a single polymer crystal, is proposed for relating birefringence, stress, and degree of crystallinity in polycrystalline elastomers under tensile stress. This expression is shown to yield results in substantial agreement with x‐ray results for the degree of crystallinity of 5°C polybutadiene when &Dgr;nc0=0.157. Birefringencedensity data on natural rubber (due to L. R. G. Treloar) are shown to be a special case of the foregoing expression, yielding a value &Dgr;nc0=0.218 for natural rubber which is in good agreement with the theoretical value for this quantity.