The nucleolar areas of fat tissue from five development stages of Drosophila mulleri were studied. Data showed variation in mean nucleolar areas for both males and females, decreasing from early third instar larvae to prepupae and increasing in young pupae. Mean nucleolar areas were significantly greater in females than in males at every stage examined. Morphological variations due to the presence or absence of cytoplasmic dense granules and fat droplets were also observed in the fat tissue cells. In males and females, dense granules were first seen in late third instar larvae and progressively enlarged. Fat droplets were seen in every development stage, but in prepupae and pupae large drops were more frequent. Results obtained in the present study paralleled data in the literature on protein synthesis in fat cells, indicating that nucleolar areas are larger in stages of high protein synthesis and smaller when that synthesis level decreases.