Polarization decorrelation in single-mode fibers with randomly varying birefringence is studied. We find that decorrelation length is minimized for a given beat length if the average autocorrelation length of the birefringence is close to the average beat length. The differential time delay between the polarization modes is found to depend on the autocorrelation length of the birefringence rather than on the decorrelation length of the polarization modes.