An alpha-driven fast magnetosonic wave instability is investigated in tokamak plasmas for propagation transverse to the external magnetic field at frequencies several times the alpha gyrorate. A two-dimensional differential quasi-linear diffusion equation is derived in cylindrical υ⊥-υ∥ geometry. The quasi-linear diffusion coefficients in the small parameter k∥/k⊥ are expanded and the problem is reduced to one dimension by integrating out the υ∥ dependence. Reactor relevant information is obtained using data from the one-dimensional formulation in a 1½-dimensional tokamak transport code. Contour plots of the alpha threshold fraction are used to identify the instability regions in the ne-Ti plane. Alpha/background electron fractions as low as 10−6 to 10−4 may trigger the instability. For a typical reactor-size tokamak, an enhancement of the fraction of the alpha energy transferred to ions by as much as 1.5 can occur for Ti = Te at 7 keV. Still, due to the rapid equilibration of electron and ion temperatures, a < 1 to 2% increase in fusion power occurs overall.