Propulsive forces generated by fish are the result of complex interactions between the water and the fish as the fish bends its body back and forth, as well as the interactions of the various materials that make up the body of the fish. While we have a relatively good understanding of body kinematics during swimming, the internal body mechanics of fish during swimming are not fully understood. In this study we collected free swimming kinematics and whole-body mechanical data from the same fish to directly compare individual swimming performance with internal body mechanics. We measured tail beat amplitude and body waveform for Atlantic mackerel (Scomber scombrus) as they swam in a flow tank over speeds from 0.2 to 1 m/s. We then measured the body stiffness using a custom bending rig. Animals were pithed and mounted on a custom built, oscillatory bending apparatus that bent the body back and forth at preset amplitudes (5°) and frequencies (2 – 10Hz) to measure whole-body mechanics. We hypothesized that mackerel would differ in their body mechanics and that these mechanical differences would contribute to individual differences in preferred swimming kinematics. Specifically, individual fish with more flexible bodies would use higher tail beat amplitudes as well as lower frequencies at higher velocities. We found that the preferred tail beat amplitude of swimming at 2 BL/s was negatively correlated with flexural stiffness (EI). Additionally, the tail beat frequency of swimming was weakly negatively correlated with EI. These preliminary results show that individual fish differ in their mechanical properties, and that these mechanical differences may contribute to differences in swimming kinematics and ultimately to performance.