Dry matter, energy, N and ash budgets of the form ingestion (I) = growth (G) + respiration (R) + feces (F) were evaluated for the larvae of E. egle (Drury) fed Asclepias syriaca L. at 27.degree. C, 87 + % relative humidity, 14 h light. Gross efficiencies of growth (100 G/I) were 14.6% (dry matter), 20% (energy equivalents), 34.5% (N) and 3.8% (ash). Net efficiencies of growth 100 G/(I - F) were 42.6% (dry matter) and 49.6% (energy equivalents). Comparison of economies of growth of E. egle with Danaus plexippus L., reared under similar conditions, suggests a difference in digestive or assimilative processes between the 2 spp. Comparison of efficiency of energy utilization and rate of growth with D. plexippus did not support the hypothesis that efficiency is inversely related to rate of energy utilization. Comparisons of efficiencies of both matter and energy utilization with other lepidopteran larvae did not support the hypothesis of greater efficiencies of resource utilization by oligophagous than by polyphagous Lepidoptera.