The energy dynamics of 2 colonies of yellow—bellied marmots (Marmota flaviventris) were studied in the Rocky Mountains of central Colorado in 1969 and 1970. The Intake—Rejecta and Maintenance—Production models, which included an analysis of seasonal variations in energy flow parameters, yielded similar estimates of population energy flow. Colony energy flow ranged from 64.0 to 94.6 KJ°m(—2)°yr(—1). Differences between colonies in annual energy flow can be explained by variations in biomass. Peak energy flow occurred at different times in the 2 colonies and the timing was related to reproductive conditions. The marmot populations consumed 94.6 to 119.2 kJ°m(—2)°yr(—1), which represented 0.8 to 3.1% of the aboveground primary production. The efficiency with which the marmot populations exploited the available net primary production was 2 to 6.4%. Seventy—one to 75% of the energy ingested by the populations was assimilated; only 77% of the assimilated energy went into maintenance of the population biomass. Tissue growth efficiency averaged 16.8%, °5x greater than typical homeotherms. The production/maintenance ratio averaged 29.6%. The marked differences in the respiration efficiencies, tissue growth efficiencies, and production: maintenance ratios between the heterothermic marmot and typical homeotherms suggest that heterothermy represents a distinct strategy in secondary production.