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We have examined the ultrastructure of post meiotic eupyrene sperm maturation in testes of prepupae, pupae, and adult Heliothis virescens (F.) males. Emphasis was placed on the structures present in the elongating sperm tail. In each young spermatid, each cell has a nucleus, acrosome derivative, and numerous mitochondria in a large volume of cytoplasm. These mitochondria coalesce to form a body, the nebenkern. As the sperm cell begins to elongate, the nebenkern divides into two mitochondrial derivatives (MDs) of unequal size. The MDs have an outer and inner membrane, and as they elongate and coil in the sperm tail, cristae develop on the inner membrane. Concurrent with cell elongation is the disappearance of most of the cytoplasm present in the cell. The axial filament has a typical insect system of 9 + 9 + 2 tubules and is present along most of the length of the sperm tail as are the MDs. A prominent system of tubules, the manchette, forms around the MDs in a later stage of development. Two extracellular structures, the satellite body and the radial mantle, also undergo a complex series of changes during the maturation of the sperm cell.