The fine structure of the fat body and associated nephrocytes of the American dog tick, Dermacentor variabilis (Say), was described in unfed larvae, unfed nymphs, and in unfed and fed adults of both sexes. The fat body consisted of one type of cell, the trophocyte. Morphological changes that occurred in the trophocytes of both sexes were dependent on feeding. The ultrastructure of feeding male trophocytes was distinct from trophocytes of feeding females. In the feeding female, the trophocyte developed an ultrastructure characteristic of cells that produce secretory proteins. A type of scalariform cell junction was found associated with rough endoplasmic reticulum of the trophocytes. Nephrocytes were closely associated with trophocytes but were not part of the fat body. Nephrocyte ultrastructure was unaltered throughout the life-stages we examined, except at the end of oviposition. Organelles in the nephrocytes were not randomly distributed, but were found in distinct regions of the cytoplasm. Slit diaphragms at the surface of the nephrocytes were extracellular specializations that had a periodic ultrastructure.