We assayed diets of Townsend's ground squirrels (Spermophilus townsendii) in a semi-arid shrub steppe in Idaho by analyzing 1,432 fecal samples during 1991–1994. Dietary composition differed between adults and juveniles but not between males and females. Sand-berg's bluegrass (Poa secunda), a perennial bunchgrass, was the most common item in diets. Diets varied among habitats that differed in plant cover. Diets also broadened progressively during the active season of this hibernating species (February–June) to include more shrub material, annuals, and other foods. A drought at the end of the active season in 1992 caused lower body mass and survival of individuals because there were fewer annual plants during the mid- and late-season periods and fewer seeds in the late-season than in non-drought years. Sandberg's bluegrass was consumed in higher proportions during the late-season of the drought year, but most of it was senescent with reduced levels of essential fatty acids and other nutrients. Among native shrubs, winterfat (Ceratoides lanata) was preferred over big sagebrush (Artemisia tridentata). Adults in habitats that lacked shrubs had a lower probability of being recaptured the year following the drought. We conclude that preferred native shrub species provided a relatively constant food source under drought conditions, compared with perennial grasses and annual plants. Perennial grasses were the most commonly eaten food, and habitats dominated by them provided a good food resource except under drought conditions. Low germination rates of annual plants under drought conditions and their short period of succulence make them a poor food source, and habitats dominated by them are unlikely to sustain viable populations of ground squirrels.