The endoparasitic isopod, Artystone trysibia, was found within a pouch-like encapsulation in the abdomen of a naturally-infected, fresh water discus fish, Symphysodon discus, imported from South America. Larval stages released from the female isopod penetrated various body sites in the albino catfish, Corydoras aeneus, where they caused mechanical damage and hemorrhage. In one case, a larva that penetrated behind a pectoral fin was observed during a 72 day period to grow and develop into an immature male isopod which was separated from the internal organs of the catfish by a host-produced capsule. The isopod maintained an opening to the outside by the continuous movement of its abdominal appendages. The capsules surrounding isopods in both the discus fish and albino catfish contained tissue elements of body wall origin suggesting that the growing isopod causes fibrous changes of the body wall which expands to form a protective invagination. A proposed life history is discussed.