The development of germ cells of peanut worms, Sipunculus nudus , proceeding with seasonal alternating was observed by optical microscope to study reproductive cycle of the population in the sea of Guangxi. Samples were co llected monthly from January 1999 to February 2000 at three sites along the coastal sea f rom Daguansha to Baihutou in southern Beihai. Observations in this study have bring to light as follow: All individuals observed were dioecious. The earliest germ cells of both males and females were small in size and appeared in clumps attached to nurse cells of motive activity . The oocytes underwent f ive stages of development: cell multiplication ( 1- 2..m in diameter) , plasmic growth ( 3- 30..m) , follicle format ion( 30- 70..m ) , jelly coat formation( 70- 160..m) and mature egg w ith germinal vescle disappeared ( ..160..m) . The spermatocytes underwent 4 stages: cell multiplication, plasmic growth, cell dif ferentiation and maturity. The germ cells were found in the coelom all the time of a year. The spawning of this population in Guangxi.. s sea area took place several times from April to September each year and peaked from May to August.