The dependence of the elastic and acousto-optic properties of lead molybdate crystals on the direction of the wave vector and the polarization of acoustic and optical waves has been studied. The effective photoelastic constants were determined by the Dixon method for various geometries of the Bragg light diffraction. The dependence of the acousto-optic quality factor on the direction of the wave vector of longitudinal acoustic waves in the symmetry plane has been studied in detail. It is shown that in this case the largest and smallest values of the acousto-optic quality factor are observed when light is diffracted by pure longitudinal waves in this plane. It is also shown that the values of the acousto-optic quality factor in lead molybdate crystals are practically the same as in paratellurite crystals.