The vegetative system of the coenocytic alga Bryopsis hypnoides has a large central vacuole which extends throughout the body of the plant. This vacuole occupies most of the volume of the thallus, leaving only a thin layer of cytoplasm appressed to the cell wall. Numerous evaginations of the vacuole penetrate the cytoplasm. In the mature parts of the thallus the cytoplasm is divided into two definite layers: the outer layer adjacent to the cell wall contains most of the organelles excluding only the chloroplasts, which are present in the inner layer next to the vacuole. In addition to the usual organelles two previously undescribed structures were observed: the first is a spherical, dense body which is enveloped by rough endoplasmic reticulum cisternae; and the second is a larger, more complex structure which has been termed the “schizogenous body.” Intracellular bacteria are present in vegetative and reproductive stages. Gametogenesis is preceded by the formation of a plug in the basal constriction of a side branch. Repeated intranuclear mitoses and plastid fissions ensue. Cleavage into gametes is accomplished by invaginations of the plasmalemma. Both male and female gametes are formed within the same gametangium but in separate sectors. The gametes are anisogamous and differ somewhat in their ultrastructure. Gamete discharge is light-triggered and the gametes are forcibly expelled.