Captain George Henry Lane-Fox Pitt-Rivers, Secretary General of the International Union for the Scientific Investigation of Population Problems (IUSIPP, 1928–ca.1942), the precursor of today’s International Union for the Scientific Study of Population (IUSSP, 1947–present), was a central figure in population science during the 1930s. With his pro-Nazi activism, anti-Semitism, Red-baiting, and failure to leave any intellectual mark of consequence, he is an idiosyncratic and in some ways unattractive subject. However, an examination of Pitt-Rivers’s role as Secretary General reveals a wealth of information about the IUSIPP as it lurched toward collapse. It casts light, as well, on the struggle for scientific legitimacy between eugenic racialists and reformers in the 1930s, a struggle in which he played a controversial and divisive role. Using a combination of newly discovered and old archival material, this article traces Pitt-Rivers’s involvement with the Union, the British eugenics establishment, Nazi population science, and far-right British politics during this turbulent decade when demography emerged as an international discipline.