The ultrastructure of gamonts in syzygy, of gametes and zygotes inApolocystissp. was examined. The gamonts looked globular (Ø 50 μm), surrounded by a wall (gamontocyst) 150 nm thick. The gamont cytoplasm was covered only by the plasmalemma but at some points there were traces of the trimembrane pellicle; the endoplasm was filled with large oval granules of amylopectin. Gametocysts with gametes in formation had thinner walls (c. 50 nm) and a dense band had become detached and followed the contours of the inner material. Some gametes were attached to the surface of the gamont cytoplasmic mass by a peduncle; other gametes were contained in large vesicles which split the endoplasm into lobes. In more aged gametocysts, gametes looked free in the space between the wall and the residual cytoplasm. This cytoplasm, reduced by the confluence of large vesicles, showed in the outer part bundles of fibrils and microtubules and in the inner part clusters of big nuclei; amylopectin granules did not pass into the gametes. Gametes formed by two sexually complementary partners were identical (isogametes), roundish (Ø 4 μm) and surrounded by the plasmalemma which formed some micropores. From a dense polar cap radiated a cone of microtubules, some of which ran tangent to the nuclear envelope wrapping the nucleus near the cell surface. Zygotes, protected by a thin wall, 20 nm, corresponding to the inner layer of the oocyst wall, were oval (6 [μm X 4 μm) with a very dense cytoplasm in which were large lipid inclusions and small granules of amylopectin.