The harvester ant Veromessor andrei Mayr is a major seed predator on annual grassland growing on serpentine soil at Jasper Ridge, N. California. Ants forage intensively during morning and evening sessions in areas surrounding nests. Activity is at its most intense in early summer, coinciding with peak seed release for most annual plant species. Ants show strong preferences for seeds of non-dominant species, notably Microseris douglasii (DC.) Sch.-Bip., but preferences alter over the season in response to seed availability. Seeds of the dominant annual species, Lasthenia californica DC ex Lindley are not foraged until later in the summer when seeds of other species are less abundant. Seedling densities and species compositions on ant nests differ markedly from surrounding areas with species relative abundances being similar to those found on gopher mounds. An exclosure experiment in areas adjacent to nests indicated that ants significantly reduced the densities of species with preferred seeds. Ants may therefore significantly affect plant distribution and abundance within the serpentine grassland.