Electrophoretic mobility and conductivity data of aqueous suspensions of rodlike tobacco mosaic virus (TMV) (length (=300 nm, diameter 2a=18 nm) are presented in a wide range of particle concentration, below and above the overlap concentration c * =1 particle/length 3 . The mobility is investigated in relation to the small ion concentration down to salt-free suspensions. At low small ion concentration a screened Coulomb interaction between the particles appears, i.e., the double layers surrounding the particles begin to overlap. This leads to a decrease of the mobility of TMV particles. Another effect which influences the particle mobility is the change in the dissociation degree of the weak surface groups on the particles which are responsible for the particle charge