The ultrasonic attenuation in SAE 4150 steel has been measured from 5 to 100 Mc at various stages in its heat treatment. The attenuation is changed by changes in grain diameter and anisotropy. The anisotropy of grains transformed to martensite is lower than pearlite, while martensite itself becomes less anisotropic upon tempering. The dependence of the attenuation upon frequencyf, grain diameterD, and anisotropy &mgr; agrees well with the theory of Lifshits and Parkhomovskii, who predict Rayleigh scattering proportional to &mgr;2D3f4for &lgr; > 2&pgr;Dand another type proportional to &mgr;2Df2for &lgr;<2&pgr;D. The anisotropy factor &mgr;2is lower by a factor of 10 for hardened, tempered steel than for iron.