Opal phytoliths are produced by plants and persist in soils as microfossils with taxonomically distinct morphology. We found phytoliths produced by the original native perennial species in soil under an annual grassland, thus providing the first direct evidence that the Californian grassland was formerly dominated by panicoid opal-producing grass. The most common panicoid-type opals were probably produced by Stipa pulchra. Frequencies of opal phytoliths from native grasses were greater at 10 cm depth than at the soil surface beneath the annual grassland. Comparison of opal phytolith frequencies from 10 cm deep at the annual site and an adjacent relict perennial grassland site suggested that the density of panicoid opal-producing native grasses on the annual site was once similar to the relict grassland.