The ultrastructure of the "primary" nucleus of the marine green algae Acetabularia (Polyphysa) cliftonii as revealed by the freeze-etching technique is described.The nuclear envelope is perforated by about 3×10(6) pores which appear arranged nearly in rows. The nucleus is covered with a layer of cytoplasm which protects it in the isolated state and resists various manipulations. This cytoplasmic layer is protected by an elementary membrane. The latter appears rough on the cytoplasmic side, and smooth on the side facing the "cell sap". The cytoplasmic layer and the surrounding perinuclear cytoplasm are connected by cytoplasmic ducts. The nucleus appears irregularly shaped owing to many invaginations in the nuclear envelope. Sometimes mitochondria are observed in these cavities.