Certain organisms such asT. politum(Hymenoptera: Sphecoidea) offer unique opportunities for the calorimetric measurement of energy budget components under natural conditions. Natural kinds, quantities, and rates of food input are easily obtained, as are total egestion and production.The positive growth phase of the wasp's life cycle is short, its entire mean consumption of spiders (1046 cal) occurring in a mean time of 4.5 days with a mean consumption rate of 233 cal/day. Despite this high consumption rate, its maximum assimilation efficiency of 76% is similar to those reported for other invertebrate predators. This is probably due to prolonged gut retention time whereby feces are retained between 4 and 8.5 days.Secretions used in the building of the cocoon required 99 cal and mean total egestion was 244 cal.Maximum gross (45%) and net (59%) efficiencies of growth ofT. politumgenerally agree with those reported for other invertebrate predators. On the whole these statistics for the latter are lower than those given for vertebrate poikilotherm predators.