A population of Great Britain pocket mice, Perognathus parvus, was live—trapped in 1967—72 on a 2.7—ha study area in south—central Washington to determine their density, reproductive performance, longevity, movements, and home range, as well as their functional role in northern shrub—steppe habitat. During 41,310 trap—nights we captured 15,386 mice representing seven species: Perognathus parvus, Peromyscus maniculatus, Onychomys leucogaster, Reighrodontomys megalotis, Lagurus curtatus, Microtus montanus, and Sylvilagus nuttallii. Pocket mice accounted for 91%, deer mice 8%, and the remaining species only 1% of the captures. Most pocket mice remained inactive underground between December and February. Adult ♂ ♂became trappable at an average date of 26 March; ♀ ♀ emerged about 18 April. Once active, adults remained trappable for an average of 60 days in years of average—to—excellent food production, and for 90 days in years of poor production. Sex ratios of trappable animals differed significantly from 1:1 in about half the sampling periods, and usually favored adult ♂ ♂. Males had scrotal testes for an average of 4 mo, with the highest proportion in April. The first signs of estrus were observed in April, the first palpable pregnancies in May, and the last pregnant ♀ ♀ usually in July. In years of high food availability adult ♀ ♀ averaged 2 litters; in average years they had 1.1 litters; and in poor years, only one ♀ in three became pregnant. The average size of three litters found in live—traps was 3.7. In 1969, a year of excellent food production, juveniles from early litters bred during late summer. Subadults were first trapped in June for an average of 40—70 days. Survival from weaning to the following breeding season ranged between 56%—58% (1967, 1970) and 80% (1968); 17%—19% survived to the 3rd yr; and 2%—3% survived to the 4th yr. The highest overwintering success was in juveniles born during the year of lowest precipitation, poorest food supply, and reduced reproduction. Differential survival affected the age distribution of animals during the breeding season. One—year—olds dominated the breeding populations in 1968 and 1970; but in 1969 only 55% of the breeders were born the preceding year. The age distribution had little apparent effect on the reproductive success of the population. Perognathus parvus ceased aboveground activities between late summer and autumn depending upon environmental conditions, sex, and age. Population size was estimated by means of the Jolly—Seber stochastic model which helped interpret population responses when components of the population were not trappable. The peak autumnal population varied about a mean of 320 animals with only a twofold variation between the estimates for the lowest (206, 1968) and the highest (437, 1969) years. Within years the greatest variation was a fivefold increase in numbers observed between the early breeding season and fall population peak of 1969. The estimated summer population size showed a high degree of correlation with the amount of precipitation falling between October and April, precipitation that largely stimulated growth and reproduction in vegetation, which in turn affected the small—mammal population. Adult ♂ ♂ had larger home ranges (1,560—4,005 m2) than ♀ ♀ (508—2,301 m2); adults had larger home ranges than subadults. Home ranges were largest in years of average—to—excellent food production and moderate population densities; they were smallest either in years of poor resources and moderate population densities or in years of average resources but high densities. Centers of home ranges shifted little between years, indicating a high degree of stability and spatial partitioning of the area. This population of Perognathus parvus displayed a regular sex—related timing of commencement of activities, a delay in appearance of subadults, and a relatively brief duration of annual activity aboveground. Collation of the data emphashized that the seemingly dense populations of pocket mice were in fact well spaced temporally throughout the year, a well as spatially, with a minimum of overlap between sexes and, more importantly, between age groups.