The ultrastructure of the sac and tail portion of the large ampullate gland and production of the silk materials in the orb web spider, Nephila clavata L. Koch, are studied with electron microscope. Large ampullate glands, the largest glands among the seven kinds of silk glands in this species, are composed of three parts which are the excretory duct, the storage sac and the convoluted tail. The wall of the sac is composed of a single layer of columnar epithelial cells. In the cytoplasm of these cells several kinds of the secretory granules, which are commonly associated with the rough endoplasmic reticula and had characteristic crystalloid, are seen. According to the morphology and internal textures of these granules, the sac portion is subdivided into proximal(connected with the convoluted tail) and distal(connected with excretory duct) portion. Between these two portions, the proximal portion is longer than the distal by four times. Silk materials, being synthesized to the shape of secretory granules, within the glandular epithelial cell of the tail portion are released to the inner cavity by the mechanism of the eccrine secretion. These secretory granules are originated from the rough endoplasmic reticula of the glandular epithelial cells, whereas no Golgi complexes has been found in any of the cells which have been examined.