The bioenergetics of annual populations of the dusky grasshopper, Encoptolophus sordidus costalis, on a natural grassland site were examined. Peak dry weight biomass of these insects reached 1.7 kg/ha (1.85 lb/at). Oxygen consumption ranged from 2.76 μl/h for first instar nymphs to 82 μl/h for adults, values that equated with an average consumption of 341 ml oxygen/m2 during the growing season. Energy values (cal/mg) of insect body tissue averaged 5.24 while those for exuviae, faeces, and plant material were 5.38, 4.08, and 4.39, respectively. The grasshoppers ingested 2% of the green shoot primary production, which, at the latitude of the Matador experimental site, was equal to 792 kcal/m2 in 1968–69. An additional 8% was cut and dropped so that a total of 10% of the green vegetation was removed by this insect species. Twenty-five percent of the ingested food was metabolized; the rest was voided as faeces. Respiration accounted for 51% of the metabolized energy. Exuviae made up 6 to 11% of secondary production. Ninety-seven percent of the potential energy of the green vegetation removed by the grasshoppers was returned to the "decomposer compartment" of the ecosystem, with only 3% removed or lost from the system.