Predation accounts for substantial mortality during the early development of many fishes. Numerous mechanisms have evolved to avoid predatory attacks. One of these is the C-type fast-start of adult fish responding to an abrupt and unexpected stimulus. This is a short-latency response in which the fish accelerates rapidly away from its position at the time of the attack. In the zebra danio Danio [Brachydanio] rerio, this is one of the earliest behavior patterns to appear during development. It can be readily elicited in embryos as early as 44 h after fertilization, 2 d before normal hatching begins. The behavior pattern is triggered by cells of the reticulospinal system located in the hindbrain. Attacks to the anterior region activate one of a pair of these cells, the Mauthner neurons, whereas attacks to the tail stimulate either a Mauthner neuron or other cells that trigger similar response patterns. In teleosts the early appearance of the reticulospinal escape system emphasizes the probable importance of predatory interactions even for the earliest stages of development.