Israel, in her 25 years of existence, received an unprecedented volume of immigration, which was the major source of her high population growth. This immigration was heavily concentrated in the first five years, 1948–1952; mass immigration of 711,000 supplemented an initial population of 630,000. Subsequently, since 1952, a very peculiar age-sex structure has developed: namely, instead of a pyramid, a wide rectangle for the younger age groups “topped” with a narrow pyramid for the older age groups. The peculiar age-sex dynamics is analyzed in relation to the volume of immigration with its uneven time distribution, the age selectivity of migrants and fertility-mortality patterns of migrants. It is concluded that the uneven time distribution of immigration and the higher fertility of migrants are jointly responsible for the development of Israel’s peculiar age dynamics, and that the absence of either of these two factors alone would eliminate it. The peculiar dynamics has societal implications in the short and the long run, some of which are discussed.