By classical electron microscopy and immunoelectron microscopy, the biogenesis of trichocyst secretory granules has been followed in the ciliated protozoan Pseudomicrothorax dubius. The very early pre-trichocysts form by fusion of bristle-coated, electron-dense vesicles (dense vesicles) with electron-translucent vesicles (clear vesicles), both of which originate in a well-developed trans-Golgi network (TGN). The pre-trichocyst grows by further fusion with dense and clear vesicles as well as with other pre-trichocysts until it reaches its maximum diameter of about 2 μm. Dense and clear vesicle formation from the TGN has been followed, and the fusion sequence of dense vesicles with the pre-trichocyst has been documented. The contents of the dense vesicles are the precursors of the trichocyst tip, which is composed of four arm-like rods, whereas the shaft precursors are supplied by the clear vesicles. The first evidence of trichocyst shaft formation is the appearance of a paracrystalline, dense core condensation center in the pre-trichocyst. Following shaft formation, the trichocyst tip forms by fusion and condensation of the dense arm precursors along each of the four sides of the shaft. Docking of the fully formed trichocyst in the cell cortex is described. Pre-trichocyst biogenesis in cells grown with and without Se is compared