Depolarized dynamic light scattering was used to measure the translational and rotational diffusion of tobacco mosaic virus, TMV, in aqueous solutions of dextran (M similar to 505 000). TMV is an electrically charged, nucleoprotein assembly with the shape of a stiff, rigid rod. Dextran is an uncharged, flexible carbohydrate polymer. The TMV was held at a fixed, dilute concentration (0.5 mg/mL), while the concentration of dextran spanned both dilute and semidilute regimes (0-14.5% by weight). There was no evidence of phase separation or strong aggregation of the TMV particles in the presence of the dextran. The TMV particles dominated the depolarized scattering at all dextran concentrations. The angular variation of the decay rates of the autocorrelation functions always followed the form expected for symmetric top molecules in the absence of translational-rotational coupling. Nevertheless, translational and rotational motions are almost surely coupled in most dextran-containing solutions. The apparent translational and rotational diffusion rates decreased with added dextran, but not exactly according to the rise in macroscopic solution viscosity. A transition occurred at about 6.5% dextran. Beyond this concentration, pronounced failures of the continuum (Stokes-Einstein) relation between diffusion and viscosity were found. Translational diffusion continued more rapidly than expected on the basis of the macroscopic viscosity, while rotational diffusion fell sharply below expectation. The quotient D-r/D-t of rotational and translational diffusion, which presumably cancels effects due to viscosity, also dropped suddenly above the transition point. These findings are consistent with a sudden onset of topological constraints to rotational motion of the TMV, without onset of severe constraints to translational motion. Temperature dependent studies showed that either the solution or the solvent viscosity can describe translation and rotation fairly well, at least at concentrations below the transition. Energies of activation for translational and rotational diffusion of TMV were similar and not strongly dependent on dextran concentration in this regime.