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Mule deer (Odocoileus hemionus) in the Sierra Nevada rely heavily on mountain whitethorn (Ceanothus cordulatus, Kell.) and deerbursh (C. integerrimus, H&A) as summer forage. In this study, mountain whitethorn leaves, deerbrush leaves, and deerbush twigs were collected from shrubs growing in full sun every 2 weeks during summer, and from shrubs growing under a range of overstory crown closures during late summer-early fall. Samples were analyzed for calcium, phosphorus, crude protein, in vitro digestible dry matter (IVDDM), gross energy, digestible energy, and sequential fibers. Summer samples of all 3 forages had adequate concentrations of calcium, apparently adequate concentrations of crude protein, and inadequate concentrations of digestible energy and phosphorus for growth and development in deer. IVDDM values were lower than expected based on fiber content alone, suggesting high concentrations of digestion-inhibiting compounds. In general, forage quality declined as summer progressed. Crown closure and shrub age had only minor effects on forage quality, but significant annual differences were found in several variables in both species. Under conditions common to the southern Sierra Nevada, annual differences in precipitation may have been more important than available light in determining forage quality. Forage deficiencies in late summer may have a substantial adverse affect on newly weaned fawns. Marginal forage quality with respect to certain nutrients suggests the need to further explore deer nutritional ecology on summer and other seasonal ranges in the Sierra Nevada. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020