The cochlear hair cell has a remarkably high sensitivity to the displacement of the basilar membrane of the inner ear. However, the detailed mechanism of the receptor process, especially for the high sensitivity, has not been fully established. The present article is concerned with the enhancement of the mechanosensitivity of the receptor hair cells of the lateral‐line organ of aquatic animals by the environmental potassium ions. The potassium ion on the receptor surface of the lateral‐line organ, the free standing neuromast, modified the receptor sensitivity to vibratory mechanostimuli. Studies on the free neuromast rather than on the canal neuromast may better contribute to the understanding of the excitation mechanism of the cochlear hair cell.