Electron microscope studies were made of wall regeneration by Pythium protoplasts. Wall regeneration began with the formation of a loose network of fibrils on the surface of the protoplast followed by increase in density of the fibrillar mesh and deposition of granular mitrix material. The majority of the protoplasts did not develop beyond the loose fibrillar network stage, however a small percentage were able to complete wall formation and to form hyphal tubes. A clear zone of demarcation was visible between the fibrillar surface of the protoplast and the smooth surface at the base of the developing hyphal tube.