We examined the accumulation of lipids in adult females of the mosquito, Aedes aegypti. Females emerged with about 100 µg lipid in the fat body. With access to sugar water lipids increased over seven days to 300 µg. After a blood meal on day five, sugar-fed females accumulated 120-140 µg of lipids in their ovaries within 2 days. At the same time the lipid content of the fat body decreased by 100 µg, indicating transfer of lipids from fat body to oocytes. Experiments in which fat body lipids were prelabelled support this conclusion. Label was transferred to oocytes: in mature oocytes the specific radioactivity of lipids was 80% of the specific radioactivity of prelabeled fat body lipids. Components of blood meals are also used to synthesize oocyte lipids. Fat bodies of females starved for four days had only 27 µg of lipids left. When these females were given a blood meal, they matured oocytes, although the number of ooyctes was reduced and ovaries contained only half the amount of lipids found in ovaries of females which had first fed on sugar water. Fat body lipids of these females had only slightly increased to 36 µg. This demonstrates that female Ae. aegypti use sugar to synthesize lipids, but they can also use components of blood for this purpose.