Density dependence is a common phenomenon in nature, and the intensity of density dependence is driven by competition over depletable resources. Habitat selection patterns are often density-dependent, and are driven by decreasing population mean fitness in a habitat as population density increases in that habitat. Yet not all resources are depletable, and non-depletable resources may sometimes be most important in dictating patterns of habitat selection. Ectotherms, for example, are defined by their dependence on environmental temperature to regulate body temperature, and temperature is often the most important resource for ectotherms. Is density dependence an important mechanism in ectotherms, especially when temperature is a limiting factor? In this thesis, I examine density dependence of fitness and habitat selection by ectotherms using red flour beetles and common gartersnakes. In chapter one and three, I test whether density-dependent habitat selection occurs when habitats differ in both temperature and food availability with red flour beetles and common gartersnakes, respectively. In chapter two, I modify the isodar model of habitat selection to account for the effect of temperature on ectotherms, derive predictions from the modified model, and test these predictions with controlled experiments with red flour beetles selecting between habitats that differ in food quantity and temperature. Finally, in chapter four, I examine the effect of density on metrics of fitness and habitat selection with common gartersnakes. Red flour beetles exhibited strong density dependence in both habitat selection and fitness at their optimal temperature, but density dependence weakened at lower temperatures. Common gartersnakes exhibited mostly density-independent habitat selection with a strong preference for warm field habitat over cool forest habitat, but exhibited some density dependence in habitat selection within field habitat. Overall, my thesis demonstrates that ectotherms have variable density-dependent responses, and that these responses are strongly modulated by temperature.