1. Extracellular unit recording was done in the brain and spinal cord of unanesthesized zebra fish larvae embedded in a drop of gelatin under a compound microscope. Startle responses were elicited either by vibrating a stylus against the ear vesicle, or by direct electrical stimulation of the spinal cord. 2. As in adult fish, the Mauthner neurons produced a prominent negative potential (0.5-1 mV) which occurred at the beginning of the startle reflex. The axonal conduction velocity was 1.7-3.7 m/s. 3. With vibratory stimulation, the startle reflex was elicited most reliably at rates of 1/min or less, though the Mauthner cells would follow stimulation rates of 1/s when the spinal cord was stimulated electrically. At this rate there was considerable fatigue of the spike itself, which became smaller in amplitude and longer in duration. 4. The auditory system functions during vibratory stimulation of the ear vesicle, and probably activates the Mauthner cells, though it was not concluded that this was an exclusive excitatory pathway during these experiments. Lateral line or purely tactile receptors might also be involved. The latency of the Mauthner spike varied inversely (range, 6-17.5 ms) with the intensity of the vibratory stimulus. Muscle contractions began 2 ms after the Mauthner spike. 5. We recorded several forms of startle behavior, including a single tail flip or a tail flip followed by regular or irregular tail movements. The Mauthner neuron almost always fired at the beginning of the response, but it never fired again during subsequent movements. This supports an earlier contention that the Mauthner cell initiates the tail flip but is not involved in iterative swimming behavior.