A single male individual ofTryxalis indicaexhibited a reversion of normal XO sex-chromosome mechanism to XY. The change also resulted in a decrease of diploid chromosome number from normal 23 to 22. A comparison of the karyotype and behaviour of neo-XY during first meiotic division related this change to the fusion of original acrocentric X chromosome, with one of the Li autosomes to give rise to the submetacentric neo-X while the homologous autosome transformed into the neo-Y.