Accurate forecasting of large earthquakes in the San Jacinto fault zone depends on the correct determination of fault segmentation. We have searched for discontinuities in the fault zone using as data geological maps of the fault traces, relocated earthquake hypocenters, focal mechanisms, slip rates, and historic large earthquake rupture zones. We identify nine principal discontinuities in the northern San Jacinto fault zone. These are characterized as structural discontinuities (bends, steps, branches, intersections), geodetic discontinuities (slip rate), and lithologic discontinuities. Most of the discontinuities have more than one defining characteristic. We also analyzed the local strain field orientation along the fault zone. Significant lateral discontinuities in the strain-field orientation are coincident with many of the structural and geodetic discontinuities we identify. Segment lengths defined from the data vary from about 7 to 35 km. This range of segment lengths is consistent with the lengths of large historical earthquake ruptures. However, evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rupture multiple adjacent segments. In these cases, maximum magnitudes may be larger than those observed historically. Because of the short historic record and the sparse palcoseismic record, probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments. Ours and previous delineations of segment lengths have varied considerably, indicating that assumed segment length is a major source of uncertainty in the recurrence calculations. This source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates. ¿ 1997 American Geophysical Union