1. A characteristic behaviour, the 'Mauthner-initiated startle response', was recorded and quantitatively analysed with high-speed cinematography (200 frames/sec) after vibrational stimulation in 11 of 13 teleost species which possess Mauthner cells. 2. The latency of the response is 5-10 msec. This behaviour has: (a) an initial phase, the 'fast-body-bend', lasting about 20 msec and consisting of a stereotyped displacement of the head and tail to one side and (b), a second phase, the 'return-flip', consisting of a non-stereotyped flip of the tail to the opposite side. 3. Within 100 msec after the start of the Mauthner-initiated startle response, most fish were displaced 0-5-1-5 body lengths from their initial position. The variability of the animal's location after 100 msec suggests that the behaviour is adaptively non-predictable; 4. In goldfish, the Mauthner-initiated startle response could also be elicited by visual stimulation. 5. We conclude that the fast-body-bend is the direct result of activation of one Mauthner cell and its spinal motor neurone pool. 6. In four species we described examples of apparently non-Mauthner initiated startle responses.