A dipolar self-potential anomaly of about 90 mV peak-to-trough amplitude and 5 km peak-to-trough length has been measured over the East Mesa geothermal field in the Imperial Valley of Southern California. The anomaly does not appear to be related to surface features. A surface field similar in form to the measured self-potential anomaly is generated by a source configuration consisting of dipolar current distributions along a series of three steeply dipping planes that are roughly coincident in location and depth extent with known faults. The source currents could be generated by the interaction of heat or fluid flow with changes in thermoelectric or electrokinetic coupling coefficients across faults. However, source potentials associated with these currents are considerably greater than those generated by estimated in situ coupling coefficients and heat and fluid flows. This implies that in situ coupling coefficients are much larger than those derived from room temperature measurements; that heat or fluid flows are much greater than those assumed for the East Mesa fields; or that some other mechanism is responsible for the generation of the anomaly.