Cattle (Bos taurus) and vizcacha (Lagostomus maximus) diets were examined monthly in the semiarid Caldenal in central Argentina. Cow-calf operations are the most important economic activities within the region. In spite of a widespread distribution of the vizcacha in Argentina, comparative studies of the diet of cattle and vizcacha are scarce. The objective of this work was to analyze the botanical composition, seasonal trends, and possible dietary overlap between cattle and vizcacha. Diets were determined by microscopic analysis of cattle and vizcacha feces collected from November 1994 through December 1995 in a shrubland community of the southern Caldenal. Grasses were the bulk of the diet for both herbivores. Piptochaetium napostaense (Speg.) Hack. was the most abundant grass in vizcacha (53%) and cattle (40%) diets. Prosopis Caldenia Burk. pods partially (34%) replaced this grass in cattle diets during late summer and fall. Consumption of P. napostaense was generally higher (13%) in vizcachas than in cattle, especially during the dry period of the study (21%). During the drier months, cattle consumed more of the less preferred grasses (48%). Forbs were poorly represented in the diets perhaps because of scarce rains and low availability. Classification and ordination techniques revealed seasonal trends and overlapping diets. A greater overlap (75%) was found during the wet period due to simultaneous consumption of P. napostaense by both herbivores. Trends in diet diversity were similar with indices generally higher for cattle than for vizcachas, especially during the dry period.