The lateral line system detects water flow, which allows fish to orient their swimming with respect to hydrodynamic cues. However, it is unclear whether this sense plays a role in the control of propulsion. Hydrodynamic theory suggests that fish could reduce drag by coordinating the motion of the head relative to detected flow signals. To test this hypothesis, we performed measurements of undulatory kinematics during steady swimming in the golden shiner (Notemigonus crysoleucas) at three speeds

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... (4.5, 11.0 and 22.0cms -1 ). We found that the phase shift between yaw angle and lateral velocity (20.5±13.1deg., N5) was significantly greater than the theoretical optimum (0deg.) and the amplitude of these variables created a hydrodynamic index (H0.05±0.03, N6) that was less than an order of magnitude below the theoretical prediction. Furthermore, we repeated these measurements after pharmacologically ablating the lateral line hair cells and found that drag reduction was not adversely influenced by disabling the lateral line system. Therefore, flow sensing does not facilitate active drag reduction. However, we discovered that ablating the lateral line causes the envelope of lateral displacement to nearly double at the envelope's most narrow point for swimming at 4.5cms -1 . Therefore, fish may use hydrodynamic sensing to modulate the lateral amplitude of slow undulatory swimming, which could allow rapid responses to changes in environmental flow.