The gland in the posterior end of Sepiella is a conspicuous structure, and is a diagnostic character of the genus Sepiella. Researches on this gland were few, with only Steenstrup¡]1879-80¡^described its morphology, and he thought it was very peculiar. One species of the genus Sepiella, S. japonica is a common species in fish markets in Taiwan; therefore, it was used to study the gland. Sasaki (1929) named the gland as the caudal gland, and it is followed here. The caudal gland is located at the posterior end of the mantle, along the midline, in between the two fins, and between the dorsal skin and the cuttlebone. The caudal gland is an egg-shaped cavity, and the walls of the cavity have many vertical folds. There is brown liquid in the cavity, and the liquid contains dark brown granules. The outer opening of the cavity is in the cleft between two fins, in the junction of fins and mantle, but closer to the ventral skin. There are iridophores in the connective tissue of the hypodermis of the dorsal skin above the caudal gland. As the cephalopod can control the activities of chromatophores, S. japonica may have the ability to control the expression of the caudal gland. There are circular muscles, posterior fin conjunctive muscle, surrounding the caudal gland, so we consider that the caudal gland can emit the secretions at well, and the muscle of the walls of caudal gland can assist to emit the secretions. Development of the caudal gland begins from the ventral side of the caudal gland. In embryo, the caudal gland is a lamellar epidermis. With growth, the front of the caudal gland evaginates and the caudal gland becomes sacciform gradually. The folds of the ventral walls of the caudal gland are more massive and complicated than those of the dorsal walls. There are many pores on the surface of caudal gland¡¦s walls. The secretion can be extruded from those pores. Histological sections of caudal glands indicated that the epidermis of the caudal gland is composed of simple columnar epithelial cells, and that the secretion is secreted from epithelial cells. There are many dark brown granules in the secretion of caudal glands. Some are spherical with a ciliary edge. Some are crystalloid. Some are transparent spherical balloons, and some are basophilic grains. The secretion also contains cells, including circular cells, cells with V- shaped nucleus, and cells with pseudopods. The secretion of caudal glands is organic, and also contains P, S, and Cl etc. The cobia juveniles fed with caudal glands or cuttlefish flesh drenched with caudal gland secretion disgorged the food immediately, indicating the cobias dislike something in the secretion. The length of the caudal gland is about 15.7¢Mof the dorsal mantel length of S. japonica. A positive trend between the caudal gland size and the dorsal mantle length was observed. The caudal gland sizes were not significantly different between male and female. The proportion of caudal gland length to dorsal mantle length of S. japonica was not significantly different between mature and immature individuals. The proportion was also not significantly different among seasons. It is concluded that defence is among the functions of the caudal gland.