Aedes aegypti is the principal vector of dengue world wide and a major vector of urban yellow fever. Despite its epidemiological importance, not much is known regarding cellular and structural changes in the fat body in this mosquito. Here, we applied light and transmission electron microscopies in order to investigate structural changes in the fat body of three groups of A. aegypti females: newly emerged, 18-day-old sugar-fed, and 18-day-old blood-fed. The fat body consists of a layer of cells attached to the abdomen integument, formed by trophocytes and oenocytes. Trophocytes are strongly positive for carbohydrates, while oenocytes are strongly positive for proteins and lipids. Ultrastructural analyses of trophocytes from newly emerged and 18-day-old blood-fed indicate that these cells are rich in glycogen and free ribosomes. Many lipid droplets and protein granules, which are broken down after the blood meal, are also detected. In 18-day-old sugar-fed, trophocytes display a disorganized cytoplasm filled with lipid droplets, and reduced numbers of free ribosomes, glycogen, rough endoplasmic reticulum (RER) and mitochondria. Following a blood meal, the RER and mitochondria display enlarged sizes, suggestive of increased activity. In regards to oenocytes, these cells display an electron-dense cytoplasm and plasma membrane infoldings facing the hemolymph. As the A. aegypti female ages, trophocyte and oenocyte cell nuclei become larger but decrease in diameter after blood feeding. Our findings suggest that the trophocytes and oenocytes remodeling is likely involved in functional changes of fat body that take place during aging and following a blood meal in A. aegypti females.