In a foundation population of BALB/c mice used to establish a colony at Waterloo in 1977, about 11% of adults showed either total absence or gross deficiency of the corpus callosum. Comparisons between parents and offspring, between progenies of different males, and between 13 separate lines established by full-sib inbreeding showed that variation in the adult corpus callosum reflected a genuine incomplete penetrance in a genetically uniform population. However, after seven generations of inbreeding, a spontaneous change occurred in one line (BALB/cWah 1), resulting in more than 50% of adults with deficient corpus callosum, among which about 20% have complete absence of callosal axons traversing the hemispheres.