The historical and recent demography of the Habbanite Jewish community from South Arabia, now in Israel, is examined in an attempt to account for the distinctive genetic constitution. They are of particular interest because of their very long period of isolation in the Yemen, where all that is known of their culture suggests a minimum of contact with neighboring groups throughout this whole period. Now they are settled in Israel an unusually detailed genetic and demographic analysis is possible. The population structure suggests recent changes in the pattern of marriage and survival, and hence of selection in the changed environment. Their demographic features represent combinations of those characteristic of primitive and of developed populations. Three features are identified as particularly affecting the genetic structure of the population - isolation, small size and the consanguineous mating pattern.