Dispersion of the ordinary and extraordinary indices of refraction have been measured systematically for wurtzitic Al Ga N epitaxial layers with 0.0≤x≤1.0 throughout the visible wavelength region. The dispersion, measured by a prism coupling waveguide technique, is found to be well described by a Sellmeier relation. Discrepancies among previous measurements of refractive index dispersion, as a consequence of different growth conditions and corresponding band gap bowing parameter, are reconciled when the Sellmeier relation is parameterized not by x but by band gap energy. © 2001 American Institute of Physics. x 1-x