Measurements of the linear (Pockels) electro‐optical coefficient of wurtzite GaN are reported. The values for the electro‐optic coefficientsr33andr31are 1.91±0.35 and 0.57±0.11 pm/V at 633 nm, respectively, in agreement with extrapolations from measured second‐harmonic generation coefficients (&khgr;33(2)=−20±6 pm/V and &khgr;31(2)=10±3 pm/V) suggesting that the dominant contributions are electronic in origin. Measurements were performed using a Mach–Zehnder interferometer with LiNbO3as a reference material. Piezoelectric effects were also observed. ©1995 American Institute of Physics.