As the United States becomes even more ethnically diverse, Americans are increasingly faced with negotiating the benefits as well as the concerns engendered by this increasing diversity. From a social psychological perspective, this presents a unique context in which to study lay ideas about the interrelationship between ethnic and national identities. Previous research suggests that although people's ideas about which ethnic groups embody the national identity tend to be inclusive, there exists a persistent and automatic tendency to link the American identity more strongly with White Americans than with other ethnic groups. Although there is ample research on the relationship between immersion in diversity and prejudicial attitudes and behavior, there is little research examining whether the influence of diversity extends to implicit intergroup biases. The goal of the current work was to explore how three different aspects of environmental diversity—the salience of White Americans, the salience of Asian Americans, and the salience of multiple ethnic groups— were correlated with the extent to which individuals implicitly and explicitly grant the American identity to Asian Americans and White Americans. Additionally, this work assessed the relationship between environmental diversity and individuals' lay definitions of the boundaries of the American identity. To empirically test these research questions, we utilized ethnic demographic data from the U.S. Census Bureau as well as individual level implicit and explicit ethnic-American associations collected via Project Implicit. With zip code as the geographical unit of analysis, we analyzed the relationships between environmental diversity and implicit and explicit ethnic-American associations using hierarchical linear modeling. Over and above individual-level demographic information, we found significant relationships between environmental diversity and implicit ethnic- American associations. Interestingly, further analyses indicated that some of these relationships were moderated by racial identity. In contrast, we found no meaningful relationships between environmental diversity and individuals' explicit ethnic-American associations. Given the increasing diversity in the U.S., as well as research suggesting that individuals' implicit intergroup biases are predictive of behavior, the current work represents a pioneer exploration of the relationship between environmental diversity and intergroup bias.