Technical Abstract: The effects of age and sex class on botanical and chemical composition of diets of cattle grazing native rangelands were evaluated in a 2-year study utilizing esophageally cannulated suckling calves, yearling heifers, mature cows, and mature steers. Samples were collected each month from June through October. Determinations were made of crude protein and in vitro organic matter digestibility and of botanical composition by microhistological analysis. Dietary diversity was evaluated using Shannon's and Simpson's indices. Yearling heifers' diets were more diverse than those of other classes of cattle. This was associated with an average dietary crude protein that was similar to calves, but greater than cows or steers. Digestibility, on the other hand, was less for heifers than other animal classes. Animal class x month interactions occurred for Simpson's index, protein, and digestibility but the 2 indicators of forage quality did not vary consistently within animal class x month. Animal class x month x year interactions occurred for Shannon's index, protein, and digestibility. Differences in botanical diversity of diets resulted in differences in chemical composition among age and sex classes of cattle grazing native rangeland during the growing season. Animals used to obtain diet samples should, therefore, be of similar gender and age as any animals whose performance is being measured.