Frogfishes are capable of three pectoral-based locomotor behaviors: walking, swimming, and jet propulsion. Most fishes have hinge-joint shoulders and small radials in their pectoral fins. Frogfish pectoral fins have three elongated radials (R1, R2, and R3) and the small scapula and coracoid bones fused to the cleithrum form a ball-and-socket joint shoulder. Frogfish are noted to have an extreme range of motion (RoM) around their shoulder joint, including potential pronation during locomotion. But little work has been done to quantify this RoM or test how frogfish use the pectoral skeleton to flip their fins. Manipulations of dissected frogfish suggests that pronation of the radials is occurring when frogfish flip their fins. It is unknown whether R1 pronates over R2 and R3 when transitioning from swimming to walking, or if another mechanism is at play. In this study, we examined the rotation of pectoral R1 and R3 around the ball-and-socket joint during locomotion using X-Ray Reconstruction of Moving Morphology (XROMM), which allows for in-vivo analysis of bone motion during a behavior. We hypothesize that R1 pronates over R3 when reorienting the pectoral fin from swimming to walking. Live Antennarius commerson were used for XROMM. Biplanar x-ray videos were analyzed in XMALab and digital bone models of the left-side cleithrum and radials from postmortem μ-CT scans were animated with Scientific rotoscoping. RoM and rotation data were collected and pronation was defined as R1 distally crossing R3. We found that long axis rotation was present in R3 while positioning and planting the pectoral fin during a fin flip. However, more data is needed to determine if pronation of R1 over R2/R3 occurs along with long-axis rotation. These preliminary results confirm that XROMM can be used to measure in-vivo skeletal kinematics of the frogfish pectoral radials and test how they are used during locomotion.