Catheterization of the dorsal aorta has proven to be a useful tool for many studies involving repeated blood sampling of channel catfish (Ictalurus punctatus) and rainbow trout (Oncorhynchus my kiss). Unfortunately, vascular differences in the hybrid striped bass (Morone saxitalis x Morone Chrysops) make catheterization of the dorsal aorta difficult and impractical. Therefore, a new catheterization technique was developed to allow serial blood sampling and intravenous injection of a free swimming hybrid striped bass. This technique involves placement of a catheter through the body wall into the sinus venosus. The catheter is stabilized and its position maintained with two external fixation devices. Patent catheters were maintained in free swimming fish for periods of two weeks, with no major complications.