Population density and associated behavioral adjustments are potentially important in regulating physiological performance in many animals. In r-selected species like the fruit fly (Drosophila), where population density rapidly shifts in unpredictable and unstable environments, density-dependent physiological adjustments may aid survival of individuals living in a social environment. Yet, how population density (and associated social behaviors) affects physiological functions like metabolism is poorly understood in insects. Additionally, insects often show marked sexual dimorphism (larger females). Thus, in this study on D. melanogaster, we characterized the effects of fly density and sex on both mass-specific routine oxygen consumption (V̇O2) and hypoxia tolerance (PCrit). Females had significantly lower routine V̇O2 (∼4 μl O2·mg−1·h−1) than males (∼6 μl O2·mg−1·h−1) at an average fly density of 28 flies·respirometer chamber−1. However, V̇O2 was inversely related to fly density in males, with V̇O2 ranging from 4 to 11 μl O2·mg−1·h−1 at a density of 10 and 40 flies·chamber−1, respectively (r2=0.58, P<0.001). Female flies showed a similar but less pronounced effect, with a V̇O2 of 4 and 7 μl O2·mg−1·h−1 at a density of 10 and 40 flies·chamber−1, respectively (r2=0.43, P<0.001). PCrit (∼5.5 to 7.5 kPa) varied significantly with density in male (r2=0.50, P<0.01) but not female (r2=0.02, P>0.5) flies, with higher fly densities having lower PCrits. An extensive survey of the literature on metabolism in fruit flies indicates that not all studies control for, or even report on, fly density and gender, both of which may affect metabolic measurements.