Freeze-etching and thin sectioning were used to examine the fine structure of the marine dinoflagellate Peridinium trochoideum (Stein) Lemm. Among the cytoplasmic inclusions described were three typical dinoflagellate organelles: a dinocaryotic nucleus with condensed interphase chromosomes, chloroplasts with thylakoids associated in groups of three, and trichocysts contained in membranous sacs. In addition to the above, dictyosomes, cytoplasmic membrane systems, fibrous bodies and 'segregated bodies' were observed and described. Upon examining the gross morphology of the theca, it was found that thecal age could be determined by the extensiveness of sutures, pits, and blisters which became predominant on the external thecal membranes with age. Characteristically, older cells sometimes had continuous, deeply scored sutures with adjacent 'marginal suture bands' and intercalary bands. Individual thecal plates of mature cells were not completely enclosed within membrane sacs as commonly assumed and some adjacent plates were found to be continuous. Four membrane systems were found to be associated with the thecal plates: the thecal membrane and outer plate membrane systems lay above the plates, the inner plate membrane and plasmalemma lay below. Sutures were formed by infolding of the outer plate membrane between adjacent plates. With the exception of the plasmalemma, the membranes associated with the plates displayed membrane asymmetry. In the mature, thecate,non-dividing cells, densely staining inclusions termed 'pro-thecal bodies' were found to be distributed throughout the cytoplasm. Before ecdysis, each amorphous prothecal body transformed into many vesicles each of which contained fibrous material in an electron transparent matrix. It appeared that 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 the new wall was formed by deposition of material from prothecal bodies at the protoplast surface. The newly formed wall was continuous over the protoplast and no plates existed as such. Pores, however, were present. The sutures, when first formed were shallow and discontinuous. / Science, Faculty of / Botany, Department of / Zoology, Department of / Graduate