The interrelations of flora and fauna of the Ohanapecosh Hot Springs, Mount Rainier National Park, Washington State, were studied during a 4-year period commencing in 1963. The flow of energy and efficiency of its transfer were measured in two springs, one with an annual mean temperature of 35 C (SD 2.0) and the other a constant 47 C. In addition to temperature differences, each spring contained a different flora and fauna.Primary production estimates were obtained together with data relative to many aspects of the natural history of the dominant dipteran herbivore Hedriodiscus truquii (Bellardi), hitherto undescribed in the larval stage, and common to both springs. Aspects of the natural history of Hedriodiscus pertinent to the energetic approach are discussed, including a detailed caloric investigation. Information about the seasonal variation in lipid composition during larval growth was used to explain much of the observed caloric variation. Energy budgets for three of the four instars of Hedriodiscus were determined from estimates of growth, respiration, and rates of food ingestion. This information together with population census data permitted calculation of population energy budgets. Efficiencies of energy transfer in both spring communities are compared and discrepancies in the allocation of assimilated energy between populations are attributed to variation in diel behavior patterns and differences in food ingested. Export of Hedriodiscus larvae and spider predation on newly emerged adults inflicted heavy mortality and were considered the most important sources of population regulation. The annual energy balance for the two springs was tabulated from estimates of energy income and export. The springs fixed far more energy than was expended in any given year. Greatest losses are attributed to algal export, respiration, and travertine deposition; tree leaves represented a significant income of energy to the springs. Primary production accounted for better than 90% of total income and only 0.5–1.0% of algal primary production was channeled to the dominant herbivore, Hedriodiscus. Of algal gross production 20–25% was exported from the system. Photosynthetic efficiency was 1.06% utilization of visible light available to algae.