Animals usually encapsulate artificial implants placed in the abdominal cavity, but some fish species are unique in exhibiting transintestinal expulsion of the implant. Gross and histological examination were used to study the mechanisms of expulsion of artificial implants from channel catfish Ictalurus punctatus. Silicone-rubber- or paraffin-coated polystyrene transmitter capsules (0.5% or 2.0% of the fish's body weight) were surgically implanted into the peritoneal cavity of 74 adult channel catfish. Within 23 d, 39 transmitters had been expelled: 14 through the intestine, 24 through the incision, and one through a lesion in the ventral body wall. Two fish died, but 33 fish retained transmitters for the duration of the experiments (14–23 d); at necropsy, four transmitters were free in the abdominal cavity, 11 had limited fibrous proliferation, 14 were fully encapsulated by fibrous connective tissue, and four were partly or fully in the intestinal lumen. A fibrous connective tissue capsule was present in all but three fish that expelled their transmitters. Contraction of myofibroblasts, a prominent cell type in the fibrous capsule tissue, provided the force for expulsion. Expulsion occurred through the incision only when the transmitter was encapsulated over the incision. Anal exits occurred when the transmitter was encapsulated on the intestine and focal inflammation weakened the intestinal tissue layers and allowed the transmitter to pass into the intestine. Tissue reactions and number of incision exits were significantly greater with transmitters 2.0% of body weight than with transmitters 0.5% of body weight. Incision exits were more frequent in gravid females than in males or spent females. The kind of transmitter coating had no significant effect on expulsion.