1. The Crucian carps (Carassius auratus) obtained in Qinpu and Songjiang districtsof Shanghai Municipality are usually heavily infected by the parasitic Isopod- Ichthyoxenus japonensis. 2. The parasites, in pairs dwell in a pouch occupying nearly one-third or half ofthe body cavity of the host, and an opening is situated near the base of the pectoral fins. 3. The fishes bearing the parasites have the gonads poorly developed. nearly 97%of them have the gonads remained at stages Ⅰ--Ⅱ, seldomly at Ⅲ. 4. The body size of the female parasite. attains a longth of 1. 4--2. 1 cm, while themale has only half the length. In the breeding season. the female moults and form a broodpouch by the flat projections of the five pairs of thoracic legs. The abdominal append-ages function as gills. 5. In Shanghai and its vicinity, the breeding season of Ichthyoxenus begins fromAprill to October. The fertilized eggs are released into the brooding pouch and developinto first stage larvae. After a moult the larvae enter to the second stage. They possessthe ability of free swimming, and than larvae the pouch to find hosts. 6. There are great difficulties in eliminating the adult Ichthyoxenus from the hostby chemical treatments, so that the preferable way is to kill the larvae in their free swim-ming stage. During the breeding season, by spraying 0. 1 p. p. m. sodium pantachloro-phenate along the littoral zone once a month seems avilable.