Fish larvae swim by undulating their body. In order to understand the role of fluid-structure interaction on the shape of the body wave, we focus on the role of body stiffness in the swimming performance of larval zebrafish. In particular, we compare the wild type with a mutant called stocksteif. In this mutant, ossification is accelerated and affects all cartilage tissue of the vertebral column, causing the vertebrae to fuse into a stiff rod over the first 15 days of larval development. By comparing wild-type and stocksteif morphs, we studied how stiffening the vertebral column affects the shape of the body wave, and how this change in body wave kinematics in turn affects escape performance. We recorded escape responses from a top view at 1500 frames per second to determine swimming kinematics. At age 5 days-before the vertebral column shows significant ossification-the two morphs' body wave kinematics and escape performance are not significantly different. At age 15 days, the vertebral column of the stocksteif mutant is largely fused. This causes angular acceleration (of the posterior body relative to the anterior body) and peak curvature to be lower in the stocksteif mutant compared with the wild-type. Both body wave parameters affect the larva's escape performance: stocksteif larvae take longer to achieve peak translational accelerations. The increasing stiffness of the vertebral column seems to seriously limit the axial muscles' ability to bend and undulate the fish's body, which in turn deteriorates escape performance.