Some plants can hyperaccumulate the element selenium (Se) up to 10,000 mg Se kg-1 DW. Hyperaccumulation has been hypothesized to serve as a defensive function against herbivory. In earlier laboratory studies, high Se levels were shown to protect plants from a variety of herbivores owing to deterrence and toxicity. However, field studies linking Se accumulation to herbivory protection are lacking. In this study a combination of field surveys and manipulative field studies were done to test whether plant Se accumulation in the field deters herbivory by black tailed prairie dog (Cynomys ludovicianus). The Se hyperaccumulator Astragalus bisulcatus (two-grooved milkvetch) occurs naturally on seleniferous soils in the Western U.S, often on prairie dog colonies. In our field surveys A. bisulcatus was one of relatively few herbaceous non-grass species thriving on prairie dog colonies. In addition, it suffered less prairie dog herbivory than other species on the colonies. This protection was likely owing to Se accumulation, as judged from subsequent manipulative field experiments. When given a choice between pairs of plants of the Se hyperaccumulator Stanleya pinnata (prince's plume) that were pretreated with or without Se, prairie dogs preferred to feed on the plants with low Se; the same results were obtained for the non-hyperaccumulator Brassica juncea Indian mustard. Plants containing as little as 38 mg Se kg-1 DW were already protected from herbivory. Together these results shed light on the functional significance of Se hyperaccumulation and the possible selection pressures driving its evolution. They also have implications for the use of plants in Se phytoremediation, or as Se-fortified crops.