The electrical and optical properties of rodlike fd-virus particles (L = 880 nm, D = 9 nm) are investigated by magnetic and electric birefringence experiments and supplemented by electric field light-scattering measurements. In aqueous suspension the fd-virus particles are negatively charged and surrounded by diffuse counterion clouds. The examinations are performed at very low ionic strength (10(-6) M). Under these conditions a liquid crystalline phase already occurs at fd-partiele concentrations above 0.6 mg/mL. In magnetic fields the rodlike fd-virus is aligned parallel to the magnetic field due to its diamagnetic anisotropy. Almost complete particle alignment in magnetic fields is obtained for liquid crystalline samples, and the saturation value of the birefringence is determined. The application of electric fields results in a polarization of the counterion clouds and a statistical orientation of the dispersed particles. In low electric fields Kerr behavior is found, providing the electric anisotropy Delta alpha(el) of a single rod. At higher electric fields the diffuse Debye cloud is partly stripped away and a complete particle orientation is hindered. Electric field light-scattering investigations yield quantitatively the decrease of the electric anisotropy Delta alpha(el) in high electric fields.