Abstract
A major branch of Mauthner's (M‐) axon has been found in larvae of Xenopus laevis (Daudin). After giving off an inhibitory collateral toward the contralateral M‐cell, the branch continues forward to the preoptic nucleus of the forebrain. Following stimulation of the M‐cell, the function of the ascending branch of its axon may be to elicit startle responses corresponding to the defensive hypothalamic responses of higher vertebrates.