The harvester ant Veromessor andrei (Mayr) is a major seed predator on annual grassland growing on serpentine soil at Jasper Ridge Biological Preserve in Northern California. Ants show strong preferences for seeds of non-dominant species, notably Microseris douglassii (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. Previous studies indicated that the ants may be very selective in their choice of seeds, and the author set out to quantify the effects of ant predation and to examine the consequent effects on plant species abundance and distribution within the serpentine grassland.