Fault-guided waves reveal a low-velocity fault segment a few hundred meters southwest of the main strand of the San Andreas Fault (SAF) system. In 2004, the San Andreas Fault Observatory at Depth (SAFOD) Main Hole was drilled 2.5 km underground and 0.7 km west of the SAF surface trace. A 3-component, 4.5-Hz seismograph was installed near the bottom of this hole. This instrument recorded fault zone guided (Fg) waves originating from earthquakes along the main SAF ~2 km north and 3 km south of the SAFOD site. This ~5 km length corresponds to a distinctive low-velocity structure imaged in 2003 using microearthquakes recorded on the Pilot Hole array. Because this structure transmits Fg-waves from the main fault, it is probably connected to the main SAF and is most likely a major, unmapped fault.