The development of anisotropy as a result of plastic deformation below the glass-transition temperature is investigated and modeled for amorphous polycarbonate. Initially, isotropic polycarbonate was subjected to different extents of plastic flow in compression, and the development of its anisotropic wave speed moduli were studied using ultrasonic wave speed measurements. Longitudinal and shear wave speed measurements were performed both in the axial and transverse directions, with respect to the axis of compression. The moduli clearly indicated the development of a transversely elastic response as a result of the uniaxial compression. The measured moduli were used to model the elastic response of polycarbonate using a model for stress that depends both on the elastic and plastic parts of the deformation. POLYM. ENG. SCL, 49:1951-1959, 2009. (C) 2009 Society of Plastics Engineers