Following opercular amputation, stages in opercular regeneration in Pomatoceros lamarckii have been described by light, transmission and scanning electron microscopy. Two to three days after amputation, the rudimentary opercular filament is invested with a delicate cuticle composed of an outer filamentous layer and an inner thicker component composed of orthogonally-arranged layers of small fibril bundles. The opercular plate is uncalcified and composed of two major components, an outer, thin, electron-dense layer and an inner, thicker component which structurally resembles that of the opercular filament cuticle. Between five and eight days, opercular plate calcification is initiated as needle-like crystallites. The structural organization of the organic components of the opercular plate show changes which are related to the onset of calcification. From 13–17 days, the opercular plate becomes heavily calcified and is composed of highly-ordered, prism-like crystals. X-ray diffraction shows these crystals to be aragonite. The structure of the cuticle remains unchanged except that the orthogonally-arranged fibril bundles aggregate into thicker fibres. Amino acid analysis of the regenerated cuticle and organic components of the opercular plate show that they differ from one another and from the normal cuticle and opercular plate. During opercular regeneration, the differentiation of the cuticle and opercular plate-secreting cells are described and the mechanisms of cuticle and calcareous opercular plate secretion are discussed.