Abstract: The mucouse epithelium of "degenerate gonad"(the urinary bladder)of Ricefield eel, Monopterus albus, was studied with transmission electron microscope. In the epithelium, six different cell types were recognized on the basis of their cell morphologies and their fine structures, but no any gonad-cells. These six were designated as superficial cells, lymph-like cells, mitochondria-rich cells, light cells, ellipticbody-rich cells, and basal cells. In addition, the rich cell organs were found in the all of these cells. But an elliptic body within the fifth cells was an undistinct cell organ in quality. The epithelial cells of the bladder were connected by a junctional complex consisted of by a tight junction, several desmosomes, and a lot of interdigitating junctions. The other some scattered desmosomes were found among the epithelial cells as well. In addition, the epithelium contained lots of obvious gaps and cavities among the cells in the contracted urinary bladder. And holes with unequal in quantity were found within the all of the six cell types, and the holes were the cross sections of the cell tunnels, which were formed when some long fingershape projectors of the adjacent cells were withdrawn. The urinary bladder of M. albus possessed some characteristic structures, such as AUM plaques, interdigitating junctions and projecters on the superficial cell free surface. It was also demonstrated that a gonad of M. albus on right side of the abdominal cavity was a composite-body of two gonads actually, and the degenerate gonad existed on the left side of the same cavity. The argument of us, in which the degenerate gonad of M. albus was putted forward to be the urinary bladder, was supported further.