The relationship between the Mauthner (M) cell action potential of the bullfrog tadpole and the rapid tail-flip was studied with electrophysiological and video-recording techniques. Single action potentials were elicited in the M-cell by vibratory stimulation or electrical stimulation of the eighth cranial nerve. These impulses were followed by a tail-flip to the side contralateral to the M-soma. Similarly, a tail-flip was produced by direct intracellular stimulation of the M-cell. Hyperpolarization of the M-soma blocked orthodromic action potentials and prevented the tail-flip. Therefore, the M-cell action potential appears to be sufficient and necessary to produce a rapid tail-flip which is associated with a naturally observed startle behavior.