Abstract. A method of separating the effects of two important determinants of body size in natural populations, temperature of larval development and level of larval nutrition, by making measurements of thorax length and wing length of adult flies is investigated. I show that at any given time variation in body size of Drosophila buzzatii from two sites in eastern Australia is determined primarily by variation in the quality of nutrition available to larvae. Throughout the year adult flies are consistently at least 25% smaller in volume than predicted for optimal nutrition at their predicted temperature of larval development. Nutritional stress is therefore a year‐round problem for these flies. Measurements of adult flies emerging from individual breeding substrates (rotting cactus cladodes) show that there is substantial variation among these substrates in the nutrition available to larvae. This method will allow study of spatial and temporal variation in the temperature of larval substrates and in the nutritional resources available to flies in natural populations.