Six types of nematocysts and their nematocytes in tentacles and acontia of the sea anemone Metridium senile fimbriatum were studied by electron microscopy. Microbasic b-mastigophores, microbasic amastigophores, and basitrichs have one fundamental feature in common: a straight, complexly-folded shaft with dense spines pointing apically. An additional resemblance between a b-mastigophore and a basitrich is the possession of a long, narrow, coiled thread bearing spines. An amastigophore is characterized by a short, looped, unspined thread and a cup-shaped granular matrix. Atrich and holotrich nematocysts have a coiled, spined tube of uniform diameter which lies in an evenly granular matrix filling the entire capsule. The above five nematocysts have three flaps at the apex of the capsule which open upon discharge, and each nematocyte possesses a flagellum with which is associated one or two centrioles and a striated rootlet. The long rootlet of the b-mastigophorebearing nematocyte passes through a circular band of fibrils surrounding the neck region of the capsule, and the short rootlet of the atrich lies in a dense fibrous sheath surrounding all but the apex of the capsule. The spirocyst differs from the other nematocysts in having a thin, ridged, singlewalled capsule; an inverted tube containing bundles of tubules; an apical disk covered only by a thin layer of granular material and the nematocyst membrane; and the absence of a flagellum in its nematocyte. Theories of excitation and mechanism of discharge of nematocysts and the function of spirocysts are discussed in the light of this and other recent studies of the fine structure of nematocysts. Special attention is drawn to the probable role of the folds in the walls of shaft and thread in increasing the length of the tube upon discharge.