The theca of Gonyaulax polyedra has been studied by light- and scanning electron microscopy. 1. The plates of “old” cells have growth rims in a regular pattern. 2. The growth rims overlap the margins of the adjacent plates. From this observation a rule of plate overlap can be deduced for the whole theca. 3. The degree of sculpture seems to correspond with the age of the plate. 4. For ecdysis, the armour opens along a line, that follows the borders of definite plates. 5. On the surface of the “naked” protoplast the borders of the abandoned plates are indicated by ridges, which are interpreted to be remnants of the sutures, i.e. joined membranes of neighbouring pellicular alveoles. 6. “Naked” cells divide by constriction. 7. During division of armoured cells, the theca ruptures. The line, along which the plates of the ancestral skeleton separate (fission line), is indicated by differences in the degree of sculpture of “old” and “new” plates.