The food habits of Thomomys talpoldes were studied in shortgrass prairie in north central Colorado. The following six plants comprised 88% of the yearly die Opuntia polyacantha (49.9%), Stipa comata (12.1%), Sphaeralcea coccinea (10.3%), Agropyron smithii (10.1%) Bouteloua gracilis (3%), Atriplex canescens (2.5%). The yearly diet consisted of approximately 67% forbs, 40% grasses, and 3% shrubs; approximately 70% of the material eaten was aboveground parts of plants (stems and leaves). Opuntia was by far the most important food. It was eaten in all seasons, but was utilized most heavily in midwinter, when it comprised 79% of the diet and probably served most importantly as a source of water. In laboratory experiments lVtalgoides thrived, using Opuntia as the only source of water. Certain ecological relationships resulting from the dependence on Opuntia by pocket gophers are discussed.