We document that the slowdown in the growth of the immigrant population in the United States since 2000 is the largest observed using Census data since 1870. Non-parametric tests reveal that the sharp decline is similar in magnitude to changes in migration growth rates that followed the two major historical regime changes in U.S. immigration policy. Migration rates are slowing across nearly all age, sex, educational and country of origin categories that we examine. We find that the stock of adult migrants under age 30 is smaller in 2015 than in 2000, a potential precursor to a declining overall stock, as was seen around the introduction of the national quotas regime in 1920. Heterogenous changes have led to slower declines for men than women, and an increase in the relative scarcity of low-skilled labor. Approximately half of the overall decline is due to falling Mexican immigration.