SYNOPSIS.An electron microscope study of regenerating trichocysts inFrontonia vesirztlosashowed that these organelles oripnate from primordial, membrane‐limited vesicles which appear randomly in the endoplasm. The characteristic pattern of development leading to the first definitive organization of the trichocyst involves the elaboration of the amorphous intravesicular substance into a series of closely packed fibrous sheets. These densely staining elements, laid down in two dimensional arrays, constitute the fundamental plan upon which the various structural subdivisions of the organelle are subsequently formed. Maturation involves, however, the re‐transformation of the distinctly fibrillar pattern of the body into a homogeneous, structureless matrix.The mature trichocyst, consisting of the tip, cap and body, is a pyriform structure bounded by a thin external membrane which is continuous with the pellicle. Extrusion of the organelle results in the transformation of the homogeneous tip and body into a slender shaft with transverse striations having a familiar periodicity of about 560 Å. During discharge the entire organelle is ejected, but the cap does not participate in the formation of the shaft. Elementary units of 120Å and 250 Å periodicities are also described in intracellularly discharged trichocysts. These observations are discussed in relation to current theories on the nature of the extrusion process.The endoplasmic origin of trichocysts inFrontoniadoes not support the classical theory of the morphogenetic pluripatency of kinetosomes insofar as i t relates to the direct derivation of these organelles from pre‐existing ciliary basa