The ultrastructure of pearl sacs of Cristaria plicata Leach in dif-ferent developmental stages were observed under electronic microscope. The epi-thelial cells of pearl sac have abundant microvilli in their apical surfaces, well-de-velaped basal membrane--inv aginated system and intercellular spaces ( latteral)，and there are a lot of mitochondria, ribosomes and Golgi bodies in the cytoplasm.During the early development, the epithelial cells and their nuclei are coluzunar,some nucleus division can be seen;lysosomes are strongly activelysis of cellular organelles occur usually, and many lamellar bodies form;most of the ribosomes are isolated granules scattered in the cytoplasm. During the latter development, howe-ver, the epithelial cells become euhoid or squamous, the form of the nuclei are irre-gular; lysasomes and the basal membrane-invaginated system are less than those in the early period, but the inteaeellular spaces become extremely noticeable.Rough exxdopiasmic reticulum increrses in number and almost all of the ribosomes accumulate as rosette configurations called polysomes. The connective tissue adjacent to the epithelium Consists of cells, including fibroblasts and quasi-canrieetive-tissue-calcium (QCTC) cells and myocytes, inter-cellular microfilaments, cell-like structures and cell pieces. There are more fibroblasts in the early period,more intercellular microfilaments and cell pieces and cell--like structures occur in the latter period.