Many ground squirrels in western USA persist in degraded grass-dominated habitats where diet choices are more limited than prior to European settlement. This study provided a rare opportunity to investigate ground squirrel diets in relatively undisturbed habitats with a wide range of native diet options. We studied the diet of the northern Idaho ground squirrel, Urocitellus brunneus, a Threatened species under the US Endangered Species Act. It currently has a small, fragmented distribution of about 20 km² in west-central Idaho. We collected fecal pellets over three years at five study sites with high vascular plant diversity (93 to 133 species) and analyzed them using a microhistological technique. Over all sites and seasons, diets were diverse (74 diet items); 45 species of forbs contributed 86.3% of the observed diet, whereas nine species of graminoids contributed 7.8%. Forb foliage was most utilized, followed by underground parts (roots, bulbs, etc.), and then flowers, whereas seeds were a minor component. Principal components analysis showed that season and age class contributed more to diet variation than sex or site. Kernel regression tracked changes in forb utilization over the active season from foliage to underground parts, and then to flowers and seeds. These data suggest that a prudent restoration strategy for ground squirrel habitat should focus on establishing and maintaining local native forb diversity, especially species with heavily utilized underground parts, flowers, and/or seeds. Legumes and composites are especially important. However, local ecotypes of short-statured grasses such as Poa secunda may also be useful.