We used microhistological analyses of fresh fecal pellets to determine seasonal diets of desert bighorn sheep (Ovis canadensis mexicana Merriam 1901) in northwestern Sonora, Mexico from April 1997 to December 1998. We identified 41 plant species (22 browse, 10 forbs, 5 grasses, and 4 succulents) in diets of bighorn sheep. We found no differences between diets of males and females, and diet diversity between sexes was similar (P > 0.05).Diet included: browse (45.7%), forbs (32.0%), succulents (17.8%), and grasses (4.5%). The consumption of succulents was higher during spring, decreased during summer, increased in autumn, and decreased in winter. Consumption of forbs was higher during winter and summer. Globemallow (Sphaeralceae spp.), desert agaves (Agave spp.), range ratany (Krameria parvifolia Benth.), buck-wheatbrush (Eriogonum spp.), foothill palo verde (Cercidium microphyllum [Torrey] Rose & Johnst.), Engelmann prickly pear (Opuntia engelmanii Salm-Dyck), desert ironwood (Olneya tesota A. Gray), and elephant tree (Bursera microphylla A. Gray) were consumed throughout the study. As biologists identify potential release sites for restoration of bighorn sheep in Mexico, studies of diet composition will provide managers with information for successful translocations.