The submicroscopical structure of yeast protoplasts regenerating the new cell wall or merely its fibrillar component was studied by freeze-etching. No relation was found between the number and distribution of plasma membrane particles at various stages of regeneration. Hexagonal arrangement of the particles was found only solitarily even in protoplasts synthesizing intensely glucan microfibrils in liquid media. The fibrillar network on protoplasts grown in liquid medium or fibrillar groundwork of the cell wall on protoplasts grown in gelatine medium were exposed only after etching on etched faces. The microfibrils did not penetrate the outer leaflet of the unit membrane, which consequently indicates that no structural relation could exist between the fibrils and the plasma membrane particles. During conversion of cells to protoplasts, plasma membrane invaginations were arranged end-to-end to form prolonged furrows which persisted until cell wall regeneration had been completed. Then the long furrows broke into short units. Thus plasma membrane invaginations appear to be loca, rigid differentiations of the plasma membrane which may migrate laterally. Neither the plasma membrane nor the adjacent cytoplasm showed signs of reverse pinocytosis. The endoplasmic reticulum, which was hypertrophic during regeneration, consisted of extensive membranes, often parallel in arrangement. The cytoplasm frequently contained groups of small globular particles without characteristic localization.