The reproductive cycle of the clam Paphia undulata (Born) has beenstudied. According to the cyctological characteristics of the developing germ cellsand the relative amount of germ cells, the annual cycle of maturation could be divi-ded into the following five stage: (1) proliferating stage,March to April; (2) grew-ing stage,April to May; (3) maturing stage, May to October; (4) spawning stage,May to October; (5) resting stage, November to February of next year. The spwaning season started from the middle of May to the end of October.There were two spawning peak in a year, i. e. the end of May and early October,while the matured sexual cells discharged in batches. Temperature was the mostimportant factor for influencing the maturation of the gonad and the spawningperiod of the clam. A hermaphrodite specimen was discovered and examined.