Ultrasonic transmitters were surgically implanted into the peritoneal cavity of anesthetized flathead catfish through an incision made along the linea alba. The incision was closed with a single row of sutures through the peritoneum and dermis. Postoperative care included holding the fish in water with 50 mg/liter of oxytetracycline for 48-72 hours. After obtaining surgical experience, neither survival nor growth appeared to be affected by the surgery or by the presence of the transmitter. Flathead catfish with ultrasonic transmitters were tracked intermittently for as long as 40 days in Lake Carl Blackwell, Oklahoma. Behavior appeared to be normal after an initial adjustment period of about 36 hours, during which time movements seemed excessive. The same procedure was also used successfully on largemouth bass and striped bass.