The epigean characidAstyanax fasciatusdeveloped a series of cave-dwelling populations in Central Mexico. The dorsal light reaction of the epigean fish was compared with that of a phylogenetically young cave population (‘Micos’ fish) and with that of the F1and F2hybrids between the epigean and a phylogenetically old cave form (‘Pachon’ fish). The experiments revealed a slight regression of the dorsal light response in the Micos fish and an almost complete regression of this behaviour in the phylogenetically old Pachon population. A genetic analysis was performed. It suggests that the dorsal light behaviour ofA. fasciatusis based on additive gene action, a minimum number of 3 to 6 genetic factors has been calculated to be involved in this system. The results are discussed with respect to the current controversy about the causes of regressive evolution. It is suggested that the observed regression of the dorsal light behaviour in the Pachon population is due to an accumulation of degenerative, selectively neutral mutations.