Abstract. Independent, absolutely calibrated measurements of the Mach number of the poloidal ion flow, the ion sound speed, and the radial electric field have been obtained in the CASTOR tokamak during electrode biasing experiments. Good agreement between the poloidal ion flow speed and the radial electric field everywhere in the scrape-off layer proves the existence of the EXB drift. Furthermore, the poloidal phase velocity of ion saturation current fluctuations is well correlated with these measurements, indicating that the turbulent structures evolve in a reference frame moving in the poloidal direction with a speed equal to the EXB drift. The fluctuations of electron temperature and density are reduced a factor of two during biasing.