If a spin wave propagates through a region of nonuniform magnetic field in which the effective magnon wavenumber equals the phonon wavenumber, it is partially converted into an elastic wave. The conversion efficiency depends primarily on the field gradient (H′) at the crossover point. Theories have been developed both for the case of weak magnetoelastic coupling and strong magnetoelastic coupling. For weak coupling the magnon‐phonon conversion efficiency &eegr;mpis &eegr;mp=H′crit/|H′| (for |H′|>>H′crit), whereas for strong coupling &eegr;mp= 1−&pgr;2exp(−H′crit/|H′|) (for |H′|<<H′crit). HereH′crit=&pgr;b22&ohgr;/cM&mgr; is a critical field gradient,b2is one of the magnetoelastic constants, &ohgr;/2&pgr; the signal frequency,cthe velocity of (transverse) sound,Mthe saturation magnetization, and &mgr; the shear modulus. It has been assumed that the dc magnetic field is applied along a cube edge of a cubic crystal and that the material is elastically isotropic. For yttrium iron garnet (YIG) at room temperature and a signal frequency of 3×109sec−1,H′crit≃5×104Oe/cm. The field gradients encountered in practice are usually appreciably smaller. Thus the strong coupling theory applies and the magnon‐phonon conversion should be substantially complete. The magnetoelastic interaction also gives rise to a reflected wave which originates in the crossover region. The amplitude of the reflected wave is much smaller than the amplitude of the transmitted wave.