The acoustic properties of PMMA poly(methyl methacrylate) deformed by tensile stress were investigated by the coherent Brillouin scattering method. The ultrasonic velocities of PMMA in steady stress-strain states were measured in the frequency range of 100 MHz to 1 GHz. The ultrasonic velocities decreased with increasing strain, and the ratio of the decrease increased with increasing strain rate. This result indicates that the crazing residues still remain in the steady stress-strain states, and the amount of the residues depends upon the strain rate. The velocity dispersion was observed around 400 MHz for virgin and deformed PMMA and reproduced with a single relaxation process. The relaxation process is assigned to the gamma-relaxation according to the dispersion map for methyl group relaxation in PMMA. The relaxation frequency and strength were independent of the applied stress and strain. The viscoelastic and plastic deformations have little effect on the gamma-relaxation. (C) 2000 John Wiley & Sons, Inc.