Single-channel seismic-reflection profiles across the Pacific margin of southern Baja California show a series of northwest-trending ridges, troughs, and faults. The most significant feature is a lineament with a trend between N42°W and N32°W extending along the slope and outer shelf for at least 400 km and possibly more than 550 km. The lineament is marked by a series of scarps, steep-sided ridges, and elongate intraslope basins as much as 17 km wide and 700 m deep. Sea-floor morphology and deformation and/or truncation of subbottom reflectors along the lineament show that it is a continuous fault zone, named the Tosco-Abreojos fault zone. We believe that this zone was a major part of the Pacific–North American transform plate boundary between 4.5 and 12 to 14 m.y. ago. Sea-floor offsets and disruption of the youngest sediments observed on some reflection profiles indicate that recent movement has occurred in some areas.