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Assimilation of cow liver by Lucilia illustris (Meigen) larvae was studied in the laboratory. The larvae were cultured from hatching to the end of development in the respiration chambers of an infra-red gas analyser (IRGA), and respiration was measured as CO2 production. Assimilation was calculated in both carbon and energy units. In this organism the Cr203 indicator method proved unsuitable for measuring food consumption. In early larval development respiration showed two linear phases, which are suggested to be correlated with the first two instars. Both the cumulative and the instantaneous net production efficiency, K2 (production/assimilation), were calculated. Integration of the latter to the final larval weight produced results similar to those obtained by direct determination. K2 varied both with temperature and with the developmental stage of the larvae (weight). Efficiency was highest during the most intense growth phase, or a little before it, then being 88 % in carbon units (90 % in energy units) at 25 to 300C. At this optimal temperature the cumulative K2 to the end of the feeding period was 82 % (84 %); in changing temperature conditions the corresponding K2 was 77 0 (79 %). The value of instantaneous concepts in bioenergetic research and the application of IRGA to measurements of animal respiration is discussed.