Stage-specific growth rates, mortality rates, energy intake, and energy expenditures are incorporated into an empirical model of energy budgeting for a population of eastern tent caterpillars,Malacosoma americanum(F.), Maryland. Of the 3,460 kJ of energy ingested by a hypothetical population of 1,000 larvae, 50.2% was egested, 34.5% was respired, 1.2%was lost as exuviae, 12.5%went into biomass production, 1.5%was used for silk production, and 0.3% went into egg production. Mortality losses at each stage were incorporated into the model. Population assimilation and growth efficiencies based on energy were as follows: assimilation (100 × assimilation/ingestion) = 49.8%, gross growth (100 × production/ingestion) = 15.3%, and net growth (100 × production/assimilation) = 30.7%.