Fluid motion in the vicinity of a vertical fault separating regions of different streaming potential coefficient can produce an external magnetic field of observable magnitude. If tectonic stress changes along the fault produce fluid motion, the magnetic field changes would be indicative of stress changes, and might precede severe fault motion. The largest component of the magnetic field is oriented parallel to the strike of fault. Magnetic field changes produced by this mechanism will have electric fields associated with them with similar time variation; this is in contrast to magnetic field changes produced by changes in susceptibility or remanent magnetization, which have no associated electric field. Experiments aimed at the detection of tectonomagnetic effects should include electric field sensors to help in determining the source of any observed variations. The hypothesis of Mizutani and Ishido (1976) that the magnetic field changes associated with the Matsushiro earthquake swarms were produced by electrokinetic currents seems reasonable.