The ultrastructure of carpospore differentiation for the red alga Cryptopleura ruprechtiana is described. Carposporogenesis proceeds through three developmental stages. After cleaving from multinucleate gonimoblast initials the terminal gonimoblast cells differentiate to produce carpospores. These young carpospores possess a large nucleus and numerous proplastids with a peripheral thylakoid. During the later stages of young carpospores starch begins to polymerize. Mucilage is formed within dilating concentric membrane bodies, thus forming mucilage sacs. The latter, subsequently, release their contents initiating carpospore wall formation. Intermediate-aged carpospores have more plastids which develop their internal thylakoid system. The endoplasmic reticulum produces granular cored vesicles. Mature carpospores have numerous fully developed plastids, large floridean starch granules and fibrous vacuoles. Curved dictyosomes produce cored vesicles and adhesive vesicles. The nuclear envelope is crenulated and a two-layered wall surrounds the mature carpospore.