Ultrasonically induced birefringence has been used to study coupled translational and rotational motions of large (100-nm) nonspherical colloidal gold particles in fluids. The effect is nonlinear in that the induced birefringence is quadratic in the ultrasonic amplitude. A theory is developed to explain the dependence of the observed signal on concentration, viscosity, temperature, particle size, and on the ultrasonic intensity and frequency. This enables us to determine the frequency dependence of the viscous drag on the translational motion of nonspherical Brownian particles. © 1986 The American Physical Society.