A survey consisting of four traverses was run with a proton precession magnetometer in Valle Chico approximately 50 miles SW of San Felipe. Running along the axis of the valley is the southward extension of the San Felipe fault, here apparently pulling apart with neither strike-slip nor vertical motion. The purpose of this study was to traverse the San Pedro Martir fault and the medial fault to determine if the fault structure is expressed in the magnetics, and if so the nature of the faulting. Three traverses across the San Pedro Martir Fault, two traverses located where the fault scarps are highly visible, a third where the fault is covered by recent alluvium. The fourth line of the survey crosses the structure in the center of the valley. Gologic models of the study area were constructed so that they would produce magnetic profiles similar to those obtained in the study and at the same time be geologically plausable. A likely model for the area of the San Pedro Martir fault is on of a magnetically susceptible volcanic flow, surrounded by relatively non-susceptible metamorphic and granitic rocks, that has been broken by the step faulting. The lack of an anomaly in profile D indicates that any subsurface structure is not associated with a lateral magnetic susceptiblity difference.