Measurements of the cable power supply voltage at the North Atlantic end of the fiber optic transatlantic telecommunications cable TAT-8 during the March 1989 magnetic storm has provided a measure of the large scale changes in the total Earth potential across the Atlantic during the storm interval. East-west potential changes as large as 700 volts (~0.12 volts/km) peak-to-peak were observed, with many smaller amplitude variations also seen. The largest variations in Earth potential occurred during and extended interval of a very intense eastward electrojet as measured by a magnetometer at the North American terminus of the cable. The eastward electrojet current probably exceeded 106 amps. The design of the TAT-8 cable power feed equipment is sufficiently conservative that even such unusually large Earth potentials as those measured during this storm were not a threat to the integrity of the communications systems. ¿ American Geophysical Union 1989