The propensity for defensive biting in turtles should vary ontogenetically. In species with plastral kinesis, the effectiveness of retraction into the shell as a defensive mechanism increases with body size. Therefore, adults should be less likely to bite and more likely to retract than are juveniles. We tested this hypothesis by measuring biting propensity and retraction propensity from an ontogenetic series of Kinosternon sonoriense (Sonora Mud Turtle), a species with plastral kinesis. As predicted, biting in K. sonoriense was restricted to younger, smaller turtles, whereas capacity for retraction was restricted to older, larger turtles.