Two resonances between spin waves and phonons are reported in the antiferromagnetic phase of GdAlO3. The stronger of these resonances is the analog to the antiferromagnetic resonance observed by Rohrer and Blazey, and is consistent with free-energy parameters determining their phase diagram of GdAlO3. The weaker line is attributed to a two-magnon, one-phonon process. These two resonances are observed with the magnetic field and the acoustic propagation vector along several symmetry axes. From their intensity the magnetoelastic coupling constants |G11|=0.7 cm-1 and |G44|=0.3 cm-1 are deduced. The diagonal elements of the elastic tensor and the corresponding sound velocities are also given. © 1972 The American Physical Society.