The relationships between the readiness of fourth instar Aedes aegypti to metamorphose and their weight, nutritional reserves and sex were examined. Both the critical larval weight required for metamorphosis and the potential weight were influenced by the temperature at which the larvae were reared, and were greater for females than for males. Caloric levels of soluble carbohydrates and soluble lipids correlated strongly with weight. The accumulations of both carbohydrates and lipids were sex-dependent and were increased in both sexes at the higher temperature, but caloric levels of glycogen at the critical weights were all similar. We suggest that weight per se is not a critical factor in determining readiness to pupate, and that carbohydrate levels may be one important causal variable.