The cytoplasmic fine structure of the leaves and rhizome of the giant coenocyte, Caulerpa prolifera, is described. Lining the cell wall, and enclosing a large central vacuole, is a layer of cytoplasm in which numerous chloroplasts, amyloplasts, nuclei, mitochondria, and dictyosomes are distributed. The plastids are characterized by peculiar vacuole-like regions in the stroma, which may represent areas of reserve polysaccharide deposition, and other areas containing a network of 25 A fibrils, possibly DNA. Often associated with mature plastids are distinct, terminal structures comprised of concentric or looped membranes enclosing a central amorphous core. Some amyloplasts are also characterized by peripherally distributed, short, densely-staining, microtubule-like elements. The cytoplasm itself is replete with unique long bundles of microtubules, which by position and orientation appear to be involved in cytoplasmic streaming. The dictyosomes are polar structures whose forming faces are intimately associated with overlying loops of endoplasmic reticulum. Numerous large electron-dense bodies of unknown composition are present in the vacuole. Aspects of fine structure and organelle distribution are briefly discussed in relation to the development of polarity and asymmetric cell shape in this organism.