Phytolith analysis was applied to several sites in a Jeffrey pine (Pinus jeffreyi Grev. & Balf.)-mixed conifer forest in the Sierra San Pedro Martir, Baja California, Mexico, to explore the hypothesis that the introduction of livestock in the late 18th century led to overgrazing of a prehistoric grass understory, resulting in changes to the prehistoric fire regime observed in the tree-ring fire-scar record. Stable soils in regions with extensive prehistoric grass cover retain a high concentration of total phytoliths and high percentage of grass phytoliths, regardless of historic vegetation changes. Phytoliths extracted from soil samples collected from several sites in the Sierra San Pedro Martir revealed total phytolith concentrations in forest soils were generally <0.5% by mass, with most <0.1%, whereas grass phytoliths were generally <10% of the total, values consistent with, the interpretation of a forest with sparse grass cover in the understory. Phytolith evidence suggests that there was minimal grass available for grazing in prehistoric Sierra San Pedro Martir forests; overgrazing a grass understory was probably not a major driver of changes in the prehistoric fire regime. © 2007 National Research Council of Canada, NCR Research Press. Abstract reproduced by permission.