We measure the extent of language assimilation among children of Hispanic immigrants. Our identification strategy exploits test language randomization (English or Spanish) of Woodcock Johnson achievement tests in the New Immigrant Survey and lets us attribute test score differences solely to test language. Students scoring poorly may be tracked into nonhonors classes and less competitive postsecondary schools, with subsequent long-term implications. Foreign-born children score higher on tests in Spanish; U. S.-born children score higher in English. However, foreign-born children arriving at an early age or with several years in the United States do not benefit from testing in Spanish.