Adult female Richardson's ground squirrels (Spermophilus richardsonii) spend 84% of their lifetime either sleeping underground at night during the active season or hibernating underground. To assess their use of subterranean sites, 46 females were radiocollared for periods ranging from a few days to 4.5 years. Adult females typically slept in 6–11 different chambers throughout the 15-week active season and used a single site for the 37-week hibernation season. For the 1st half of the active season, which encompassed mating, pregnancy, and lactation, squirrels usually slept in 5–6 different chambers, with each sleep site generally used once for a prolonged period of consecutive nights. In contrast, in the latter half of the active season, females moved back and forth between 5–6 sleep sites. For both females that did and did not wean a litter, this switch in sleep-site usage occurred abruptly, about 40 days before entry into hibernation. Females always slept in the future parturition chamber for some or all of gestation, and they always continued to sleep in the parturition site for some or all of lactation. In contrast, adult females never slept in the future hibernation site on any night during the preceding active season. Only 50% of the females used the hibernaculum for sleeping after hibernation, and then only for 1–5 nights. The marked contrast in prior usage of the parturition site and hibernation site indicates that the hibernaculum chamber of adult female Richardson's ground squirrels is dedicated to the forthcoming hibernation period, whereas the parturition site is a multipurpose chamber used for sleeping during gestation, for parturition, and for sleeping and litter rearing during lactation.