The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of [11 2- 0] oriented A -plane wurtzite InN films grown on R -plane (1 1- 02) sapphire substrates using molecular beam epitaxy. For A -plane films the c axis lies in the film plane. The PL signal collected along [11 2- 0] with electric vector E⊥c is more than three times larger than for E∥c. Both PL signals peak around 0.67 eV at 10 K. The absorption edge for E∥c is shifted to higher energy by 20 meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7 eV at 10 K. © 2006 American Institute of Physics.