The Chiricahua fox squirrel (Sciurus nayaritensis chiricahuae) is an uncommon subspecies endemic to the Chiricahua Mountains of southeastern Arizona. We monitored annual cycles of body mass and reproduction to elucidate factors shaping the ecology of the sexes. Body mass of adult males did not fluctuate seasonally, although males tended to be heavier in winter. Body mass of adult females fluctuated seasonally, with lower body masses in summer than winter. Males and females did not differ in body mass in summer and fall, but females were heavier than males in winter and spring. Males with scrotal testes were found in the population during all seasons, but were especially prevalent in winter and spring. The majority of lactating females were present in spring and summer. Annual cycles in reproduction and body mass of Chiricahua fox squirrels are similar to more widespread species of tree squirrels, although males did not exhibit typical body mass fluctuations. Extreme spatial and temporal fluctuations of food experienced by Chiricahua fox squirrels might result in annual patterns in space use and body mass that differ from tree squirrels living in forests with a greater abundance of food.