We study the in-plane polarization of wurtzite GaN/(Al, Ga)N multiple quantum wells. Identical M-plane (11¯00) and C-plane (0001) structures are grown by plasma-assisted molecular-beam epitaxy on γ-LiAlO2(100) and 6H–SiC(0001), respectively. While the emission from the conventional  oriented wells is isotropic within the growth plane, we observe a strong polarization anisotropy of over 90% for the M-plane sample. The luminescence is polarized normal to  and shows no spectral shift with polarization angle, i.e., it originates solely from A excitons (px and py valence band states). The deviation of the polarization degree from unity is attributed to the mixing with pz valence band states due to quantum confinement. © 2000 American Institute of Physics.