One of the methods in use for measuring the kinetics of isothermal crystallization of polymers is the recording of the depolarization of plane-polarized light by a microscope specimen. A relation between this depolarization and the volume fraction crystallized has been derived taking into account the polycrystalline nature of the specimen and assuming uniaxially birefringent crystallites. Uniform, all-directional increase in size of the crystallites would cause the depolarization to follow an Avrami-type equation with the Avrami exponent one unit “too high.” For spherulitic growth the correct Avrami exponent is found on the assumption that the birefringent entities in the spherulites do not increase in size but only in number. The depolarization by a polycrystalline specimen of uniform particle size is proportional to both specimen thickness and particle size. The derived relations are tested by random number calculation and their limits of reliability are indicated accordingly.