Behavior of the fish has been investigated so much. However, the effects of the side-wall on the fish behaviors have not been investigated. In this study, the swimming behavior of isolated ayu in a rectangular open-channel flow was recorded with a digital video camera with changing the velocity. It was found that the fish behavior is affected by the side-wall where the distance from sidewall is less than one time of body length of ayu, so that this area was defined as the side-wall area. The mode value of the ground distance, ground speed, swimming distance and swimming speed in the side-wall area is smaller than those in the universal area, and also that the angle of refraction in the side-wall area is constant, irrespective of flow velocity.
