The patterns of sex ratio in Israeli twins by maternal age and parity are described in two ethnic subpopulation. Jews and Bedouins differ one from the other in genetic, environmental, lifestyle, and reproductive factors. In the Jewish subpopulation, the proportion of males was significantly lower in twin than in singleton births; parity increased the odds of male twin births while maternal age had the inverse effect. In the Bedouin subpopulation, the sex ratio did not differ significantly from that of singletons, and no consistent patterns were found by maternal age and parity. The data suggest that several factors may influence the sex ratio in twin births. In Jewish twin births, the findings are consistent with the literature and can be largely explained in terms of high level of maternal gonadotropins. In the Bedouin subpopulation, paternal influence which could be related to lifestyle may be present, and as yet undefined genetic factors may also be involved. It would be of great interest, therefore, to continue monitoring of the effects of changes in lifestyle on the Bedouin population in order to tease out the relative importance of the varying factors on the sex ratio of twin births.