Vestibular type 1 hair cells in the waltzing guinea pig contain needle-shaped inclusion bodies which grow in an uncontrolled fashion associated with the destruction of the cell. The needles are shown to be composed by filaments of actin, a protein identified in the electron microscope by its ability to bind subfragment 1 of myosin. Whereas actin filaments in stereocilia are oriented down towards the cell body, filaments in the needles point up towards the cuticular plate. The hereditary lesion appears to be associated with a defective control of polymerization of actin into filaments.