Solutions of the bacteriophage R 17 have been studied by time-dependent light scattering, using a multichannel digital correlator to measure the photocount auto-correlation function. The zero-concentration limit of the diffusion coefficient was D020.w = 1.540 ± 0.015 x 10-7 cm2/s. This value was used to calculate the hydrodynamic radius, Rh = 139 ± 1,4 Å, and, in combination with literature values for the sedimentation coefficient and partial specific volume to yield a molecular weight, 3.81 ± 0.14 x 106, and the degree of solvation 1.11 ± 0.13 cc solvent per gm virus. At higher virus concentrations, D20,w is a function of both virus concentration and solution ionic strength. This effect is explained in terms of screened interparticle electrostatic interactions. At high ionic strength an effective excluded volume dependent on ionic strength can be defined, whereas at low ionic strengths the interactions appear to become long range leading to unusual angular dependences of both the magnitude and time-dependence of the scattered light