Scholars recognize Czech-American Aleš Hrdlička (1869–1943) as a founding father of physical anthropology in the United States. Most recently, Samuel Redman’s Bone Rooms (2016) devotes many of its pages to Hrdlička. As the Curator of Physical Anthropology at the Smithsonian Institution from 1910 until 1941, Hrdlička played an authoritative role in defining racial categories in America. This paper uses fresh archival research, including previously neglected Czech-language material, to demarcate the borders Hrdlička set for the white race. As America’s leading expert on race, Hrdlička often gave friendly advice to ordinary people who wrote to him with their questions about racial categories. In some cases, Hrdlička’s judgments about racial identity had weighty legal consequences, as in the controversial White Earth litigation (1916–1920) concerning land allotments to Anishinaabe Indians. This chapter examines Hrdlička’s pronouncements on four specific groups: Finns, Jews, Slavs, and Koreans. Curious citizens hoped that Hrdlička could use science to clarify the murky boundaries of the white race. However, his conception of whiteness reflected little more than his own personal priorities and cultural values.