Variations in the morphology and biochemical content of insect fat body have been associated with metabolic activity and the reproductive cycle (synthesis of vitellogenin). In social insects such as bees, the functional traits of fat body also differ between workers and queens. In this work, we used light and transmission electron microscopy to examine the morphological features of fat body trophocytes of virgin and physogastric mated queens of the stingless bee Melipona quadriafasciata anthidioides before and during vitellogenesis. Virgin queens had few, small fat body cells in which lipid deposits predominate, and showed no evidence of biosynthetic activity or the uptake of exogenous substances. In contrast, the fat body cells of physogastric queens were almost completely devoid of lipids, exhibit a well-developed rough endoplasmic reticulum with an obvious intraluminal product, and contained Golgi stacks that release numerous vesicles. These ultrastructural findings were suggestive of proteosynthesis. However, there was no evidence for the accumulation of synthesized material in the form of secretory granules. We conclude that the trophocytes of virgin and physogastric queens differ basically in their switch from a storage role in the former to a synthetic role in the latter. In addition, the high level of vitellogenesis seen in egg-laying queens suggests that the main material synthesized is vitellogenin.