An inherited exencephaly in the mouse is described, using embryos at 11–17 days gestation. The cephalic portion of the neural tube remains open on the dorsal surface, the bony vault of the brain fails to develop, and the tissues of the brain are exposed; the development of these tissues, which is somewhat variable, is described. There is strong evidence that the condition is due to a single recessive gene, xn, with full viability until birth. Penetrance is twice as high in females as in males, and it varies overall according to the genetic milieu studied, from 33% to 84%. These features, and the absence of pleiotropic effects and of genetic evidence of associated chromosomal anomalies, make its inheritance different from that of exencephalies so far described.