The fat body of the female stick insect,Bacillus rossius(Rossi), was examined ultrastructurally in developing larvae. Two different fat body tissues are present in first instar larvae: A remnant of an embryonic tissue, and a newly differentiated tissue appearing only after hatching, the larval fat body. The embryonic fat body consists of large cells with numerous lipid droplets of saturated fatty acids. They are randomly dispersed in a dense cytoplasm packed with free‐ribosomes. The larval fat body includes cells highly enriched in glycogen and with numerous unsaturated lipid droplets. With the proceeding of development, the embryonic fat body is destined to disappear. By contrast, the larval fat body undergoes progressive decrease in glycogen content, and the cytoplasm gradually fills with numerous Golgi apparatus and cisternae of the rough endoplasmic reticulum. Lipid droplets remain in the larval fat body until the transition to the adult stage takes place. With the attainment of the stage II of development, composite secretory granules appear in relevant number in the cell cytoplasm. Initially, they retain a random distribution in the cell cytoplasm, but later move to the apical cell surface by stage III. By stage IV, the fat body cell surface comes to be characterized by an extensive plasma membrane reticular system that projects onto the basal lamina. Following transition to the adult stage and disappearance of the lipid droplets from the tissue, the cell cytoplasm comes to manifest a clear apico‐basal polarity. Composite secretory granules are thus seen to pile up in the apical cytoplasm, following a progressive sequence of maturation from the Golgi apparatus. These observations allow the conclusion that fat body cells inB. rossiusmake use of the same secretory pathway leading to formation of composite secretory granules, irrespective of the haemolymph protein being secreted during larval development.