The rotational diffusion coefficient of fd-virus particles was determined by electric light scattering decay measurements. The value obtained by extrapolation to zero concentration is in good agreement with that obtained earlier by other methods, The saturation electric light scattering data are lower than the theoretically expected, This has been interpreted as an indication of the considerable flexibility of the fd-virus, The electrostatic and hydrodynamic interactions of the particles, whose nature was tested by the concentration and ionic strength dependence of the decay relaxation time lead to maximum structuring of the system at about 5 C*, The understanding of the concentration dependence of the electro-optic effects could be a good basis for more successful application of other electro-optic methods that give more information on the structuring of the samples such as electric birefringence and especially electric dichroism. On the other hand, the data on electrostatic interactions as a function of the interparticle distance (i,e., virus concentration) might be useful for the understanding of the infection mechanism.