The lateral‐line organ of killifish is shown to be sensitive to a linear function of water displacements associated with the near‐field of sound sources, with the displacement probably being the most important factor rather than velocity or acceleration. The near‐field effect is discussed and is shown to be important not only for the lateral‐line organs but also for the acoustical and vestibular organs. It is emphasized that the near‐field effect introduces considerable complications into the study of the acoustico‐lateralis system, and is of conceptual importance for the theory of hearing and the study of schooling fish.