Tree ring fire-scars in Sequoiadendron giganteum (giant sequoia) stands record a high frequency, low intensity, prehistoric fire regime. Difficulties achieving short prehistoric fire return intervals with prescribed burns at a S. giganteum stand in Calaveras Big Trees State Park, California, currently characterized by dense tree cover with little understory vegetation due to over a century of fire suppression, suggest that a prehistoric grass understory provided fine fuel required for frequent fire spread. We used phytolith analysis to test this hypothesis. Phytoliths, microscopic silica bodies found in many plants but produced in large quantities with distinctive morphotypes in grasses, are preserved for thousands of years in the soil. Soils under vegetation with extensive prehistoric grass cover retain a high concentration of grass phytoliths regardless of historic vegetation changes. Phytoliths were extracted from soil samples taken from pits dug at 14 plots throughout a S. giganteum stand in the South Grove Natural Preserve. Soil phytolith weight for most plots, currently without grass cover and comprising most of the stand, was less than 0.10%, consistent with reported values for forests with no grass in the understory. Soil phytolith weights for ridge-top plots and plots near the stream channel were significantly higher, suggesting localized areas with sparse grass cover. The hypothesis that there was substantial prehistoric grass cover in this S. giganteum stand was rejected.