Research on the evolution of immigrant fertility patterns has focused on the expected reduction in fertility among immigrants from high fertility, less developed countries who arrive in relatively low-fertility developed societies. The current research considers a different context in which immigrants from the low-fertility Former Soviet Union arrive in a relatively high-fertility setting in Israel. This research context allows us to test various theories of immigrant fertility, which cannot normally be distinguished empirically. Results from Cox multivariate regressions of parity-specific progression do not support assimilation theory, which would predict an increase in fertility following migration, in this context. We interpret the very low fertility rates of the FSU immigrants in Israel, relative to all relevant comparison groups, in terms of the economic uncertainty and hardship experienced during a difficult transition period by immigrants who have high aspirations for social mobility in their destination society.