Stereotype threat--a situational context in which individuals are concerned about confirming a negative stereotype--has been frequently shown to test performance, with one hypothesized mechanism being that cognitive resources are temporarily coopted by intrusive thoughts and worries, leading individuals to underperform despite high content knowledge and ability (see Schmaeder & Beilock, 2012). We test here whether stereotype threat may also impact initial student learning and knowledge formation when experienced prior to instruction. Predominantly African American 5th grade students provided either their race or the date before a videotaped, conceptually demanding mathematics lesson. Students who gave their race retained less learning over time, enjoyed the lesson less, reported a diminished desire to learn more, and were less likely to choose to engage in an optional math activity. The detrimental impact was greatest among students with high baseline cognitive resources. While stereotype threat has been well documented to harm test performance, the finding that effects extend to initial learning suggests that stereotype threat's contribution to achievement gaps may be greatly underestimated. [This paper was published in "Cognitive Science" (EJ1172751).]