Ecdysteroid concentration determined by RIA in certain organs (hepatopancreas, hemolymph, eyestalk, muscle, heart, etc) of prawn (Penaeus orientalis) demonstrated that the concentration of ecdysteroid in prawn collected from Jiaozhou Bay, Qingdao in September and October is generally higher than that in the matured prawn collected in May. The concentration of ecdysteroid in eyestalk of the prawn during the moulting period is the highest. The ecdysteroid concentrations in the organs excepting muscle during the intermoult period increased gradually, and reached the highest during the moulting period. This result suggests that the activity of eyestalk Y-organ changes within moult cycle. The present experiment shows that concentration of ecdysteroid in the inclusions of midgut is 2.27一28.36 pg/mg, which suggests that there is rich ecdysteroid in the food ingested by prawn from natural sea.