In the embryonic zebra fish as early as 40 hr after fertilization, the Mauthner cells (M-cells) initiate an escape response, elicited by tactile-vibrational stimulation. The initial part of this behavior is similar to the acoustic startle reflex seen during the larval stage which begins at 96 hr. The embryonic response is directional and is followed by a series of strong tail flexures which are more pronounced than those during swimming. In the embryo the M-cell fired at the beginning of the response and rarely fired again during subsequent contractions; in our experiments the M-cell did not mediate iterative movements of the tail. The M-cell system is probably involved in evoked hatching behavior, as the tactile response is sufficient to rupture the egg membrane and allow the animal to escape. The M-cell sometimes fired spontaneously, which suggests that it might function also in spontaneous hatching behavior which occurs in the absence of phasic stimulation. At 48 hr the M-cell has morphologically mature synapses on its soma and dendrites, but its cytoplasm is relatively undifferentiated; it has few oriented neurofilaments and no distinct axon hillock. During these stages the extracellular M-spike is longer in duration and smaller in amplitude than at later times when the cell is more mature morphologically. Our data suggest that long-term inhibitory control of the M-cell system begins to function at about the time of hatching. At this time the cell is morphologically mature and is richly supplied with synaptic endings over its soma and dendrites.