The formation of the cell wall in the marine dinoflagellate Peridinium trochoideum was studied using light and electron microscopy. In mature, interphase cells, densely staining inclusions termed ‘prothecal bodies’ were found distributed throughout the cytoplasm. Before ecdysis each amorphous prothecal body developed into many vesicles, each of which contained fibrous material in an electron-transparent matrix. The vast number of vesicles so formed may have increased the cell's osmotic pressure enough to initiate ecdysis. At ecdysis the thecal plates and overlying membranes were lost and a new wall was formed by deposition of intact prothecal vesicles at the protoplast surface. The newly formed wall was continuous over the protoplast and no plates existed as such