SUMMARYLight and electron microscopy indicate thatPeridinium balticumpossesses 2 Feulgen‐positive, membrane‐bound nuclei which divide synchronously. One nucleus has the typical structure of dinocaryotic dinoflagellates, while the other nucleus has a structure typical of eucaryotic organisms. Connections between each nucleus and the endoplasmic reticulum are common. Membrane‐bound vesicles are intimately associated with the nuclear envelope of the eucaryotic nu