We discuss the problem of roto-translational diffusion of a rigid biaxial molecule dissolved in a uniaxial smectic liquid crystal phase. We examine distorted rod and disklike molecules and show how biaxiality and roto-translational coupling can produce significant effects on some of the correlation functions and spectral densities most useful in analyzing experimental observables. (C) 2000 American Institute of Physics. [S0021-9606(00)50710-7].