Three high-resolution seismic reflection lines were acquired in the northern part of the San Jacinto graben. The graben, a pull-apart basin formed by a dilational right step of the San Jacinto fault zone, has been previously interpreted as a simple rhombochasm. The reflection survey located at least one significant and previously unidentified intrabasin fault, referred to here as the Farm Road strand. This fault lies approximately halfway between the Claremont and Casa Loma strands of the San Jacinto fault zone. At the north end of the basin, the southwestern boundary of the graben is interpreted to be the newly identified Farm Road strand and not the Casa Loma strand as was previously thought. The identification of this intrabasin fault allows us to infer that the San Jacinto basin comprises coalescing subbasins and is not a simple pull-apart basin with an unusually large length:width ratio. The distances between the en echelon Casa Loma, Farm Road, and Claremont strands are between 1 and 2 km. This close spacing would likely permit an earthquake rupture to jump between strands and thus propagate through the San Jacinto basin. ¿ American Geophysical Union 1995