Experimental magnetooptical diffraction spectra produced by the band domain sructure in an epitaxial ferrite-garnet film are described. Asymmetry of the optical field in symmetric maxima is observed when a laser beam is normally incident on the sample. Analysis of the experimental data shows that the factors responsible for disrupting the symmetry of the magnetooptical diffraction spectrum include the inclination of the domain walls and inequality of the Fresnel coefficients fors andp polarizations of the incident radiation.