The effects of both constant (up to 2 × 10 3 V/m) and high-frequency alternating fields (up to 2 1 × 10 5 V/m, 200 kHz to 2 MHz) on suspended microtubules are investigated. At pH 6.8 and 120 mM ionic strength, constant fields cause a motion of microtubules toward the anode. The electrophoretic mobility amounts to 2 6 × 10 −4 cm 2 /Vs, reflecting a negative net charge of approximately 0.2 elementary charges per tubulin dimer. The moving microtubules are randomly space oriented. Alternating highfrequency fields induce electric dipoles and align the microtubules parallel to the field direction. By determining the angular velocity of the turning microtubules, we estimate a dipole moment of roughly 34,000 Debye at 2 1 × 10 5 V/m and 2 MHz. By comparing the potential energy of the dipole in the applied field with the thermal energy of microtubules, we obtain a minimum value of 6,000 Debye as necessary for an efficient alignment.