Polarization modulation instability (PMI) in birefringent photonic crystal fibers has been observed in the normal dispersion regime with a frequency shift of $64\mathrm{THz}$ between the generated frequencies and the pump frequency. The generated sidebands are orthogonally polarized to the pump. From the observed PMI frequency shift and the measured dispersion, we determined the phase birefringence to be $5.3\times {10}^{-5}$ at a pump wavelength of $647.1\mathrm{nm}$ . This birefringence was used to estimate the PMI gain as a function of pump wavelength. Four-wave mixing experiments in both the normal and the anomalous dispersion regimes generated PMI frequency shifts that show good agreement with the predicted values over a $70\mathrm{THz}$ range. These results could lead to amplifiers and oscillators based on PMI.