SYNOPSIS. The eye is an extraordinary organ in terms of its development and evolution. In cave animals, the eye is sometimes reduced or eliminated as a consequence of adaptation to life in perpetual darkness. We have used the characid teleost Astyanax mexicanus as a model system to investigate the mechanisms of eye degeneration during the evolution of a cave vertebrate. Eyed surface populations of Astyanax entered caves during the Pleistocene, and their descendants lost their eyes and pigmentation. Astyanax populations exhibiting various degrees of eye regression have been reported in 29 Mexican caves. Surface populations with characteristics of the ancestral stock still exist in the vicinity of these caves. Thus, Astyanax represents one of the few instances in which the ancestral (surface fish) and the derived (cavefish) developmental modes are extant and available for comparative studies. The cavefish embryo develops an optic primordium consisting of a lens vesicle and optic cup but the rudimentary eye arrests in development and degenerates. Here we report that eye degeneration is accompanied by extensive apoptosis and downregulation of the Pax-6 gene in the developing lens. The results suggest that alterations in lens development are important factors in eye regression during cavefish evolution.