Annual activity patterns and changes in mass of Wyoming ground squirrels (Sper-mophilus elegans elegans) were studied in Colorado from 1978 to spring 1981. Dates of emergence differed among years and among sex and age cohorts, and were dependent both on endogenous rhythms and on exogenous factors such as temperatures and snow cover. Adult males emerged during March, 16–18 days before females. Females were bred within 1–4 days after emergence. Both yearling males and females bred during 1979 and 1981, but yearling females did not wean litters during 1980 when ground squirrels emerged late and at low body mass because of low temperatures and deep snow cover. Parturition occurred in late April or early May, and juveniles appeared aboveground in late May or early June. In contrast to most other ground squirrel species, adult females immerged 1–1.5 weeks before adult males (late July and early August, respectively). Juvenile females immerged before juvenile males. An endogenous rhythm of consumption interacted with high body-fat levels to induce immergence in adults, and with low temperatures and low food availability to induce immergence in juveniles. Mean masses for males were significantly greater than for females upon emergence and through the breeding season. Breeding males lost mass after emergence, gained mass at a rapid rate from May through mid-July, then gained little extra mass until immergence. Breeding females gained mass after emergence, lost mass for 1 month after parturition, then gained mass again before immergence. Loss of mass during hibernation averaged 42% for adult and yearling males, 41% for adult and yearling females, 38% for juvenile males, and 37% for juvenile females. Because adults are aboveground for a short period, the amount of temporal contact between adults and juveniles is small and this species is classified as one of the least social ground squirrels.