Channel catfishIctalurus punctatusencapsulated implanted dummy transmitters in fibrous granulation tissue, Thirteen to 23 d after surgery, the fibrous capsules, 0.1–2.0 mm thick, contained numerous elongate fibroblasts (myofibroblasts) and dense collagen fibers oriented parallel to the transmitter surface, Cell types in the fibrous capsule identified with light and electron microscopy included red blood cells, endothelial cells, vascular pericytes, tissue lymphocytes, tissue and circulating neutrophils, circulating eosinophils, macrophages with ingested red blood cells, and fibroblasts with numerous intermediate filaments. The high concentration of actin in the fibroblasts, demonstrated by immunoperoxidase techniques, identified these cells as myofibroblasts. Based upon differential cell counts, macrophage concentration was higher and myofibroblast concentration was lower in the capsule area adjacent to the transmitter than at the exterior of the fibrous capsule, When a transmitter was expelled through the original abdominal incision to the environment or through the intestinal wall to the intestinal lumen, significant changes in cell numbers occurred in the fibrous capsule but only in the portion newly exposed to the environment or lumen. Lymphocyte and macrophage concentration increased after exposure to intestinal contents, whereas exposure to the external environment resulted in the formation of multinucleate giant cells.