The eyeless-Dominant (eyD) mutation is a fourth chromosome insertional translocation which affects the eyes, antennae, ocelli, and sexcombs when heterozygous but is a larval-pupal lethal when homozygous. By use of a crowding technique, it was possible to separate eyD homozygotes and heterozygotes with 100% accuracy at an early stage of larval development. Under these conditions, the eyD homozygotes had a biphasic lethal period since 45% died as first or second instar larvae and 55% died as pupae. The eyDleyD pupal lethal, isolated by this technique, was able to form all the adult structures with the exception of the eye-antennal disc derivatives. The brain was present but abnormal. In testing the competence of the eyDleyD larval eye discs by means of transplantation experiments, it has been demonstrated that the mutant discs develop non-autonomously. Histological studies have revealed the existence of neurological defects in both eyD heterozygotes and homozygotes. The eyDleyD lethal larvae lacked detectable optic formation centers and showed an extreme reduction in the number of cells present in the cortex of the brain. The ey"dleyD lethal pupae possessed partial formation centers and also had severe reduction in the number of cortical cells. The eyD heterozygotes possessed normal appearing formation centers but they did exhibit a moderate reduction in the number of their cortical cells as compared to wild-type. These studies have shown that there is a direct correlation between the extent of neurological damage and the time of developmental arrest. It appears that the eyD mutation must adversely affect the neuroblasts at a very early stage of development. As a working hypothesis, it is suggested that the eyD mutation operates via the same basic mechanism of cell death in heterozygous and homozygous animals.