This dissertation documents and analyzes the social processes by which the Mexican-origin population in the United States has been racialized. Racial formation and world system development are emphasized as interdependent in that process. It presents the social change fostered by railroad construction, which incorporated Mexican laborers into the meta-industrial system of production in the late nineteenth and early twentieth centuries. The Mexican-origin population was racialized in newspaper editorials, local segregation statutes, and in race "science." The eugenics movement portrayed them as antithetical to social hygiene. Delousing experiments were conducted on Mexican entrants to the United States at El Paso and in railroad camps in Southern California. The medical discourse heightened fears of a Mexican "invasion." New test cases in the late 1920s/early 1930s tried to prevent Mexicans from immigrating and naturalizing and a new U.S. Census in 1930 featured a "Mexican" category under "race." This is a departure from the formal classification of Mexicans as "white" from the 1850 to 1920 censuses. Nonetheless, their official racial status was debated and contested by those with competing interests in the use of Mexican labor in the United States, as well as by capitalists with investments in Mexico and Latin America at a time of violent instability in the hemisphere. Also featured are examples of how Mexicans attempted to counter racism and struggle for equality.