Most discussions of the use of race and ethnicity in biomedical research and clinical care focus on broad national and transnational populations. Looking at the problem from the perspective of large populations, however, misses the rest of a continuum that runs from the global human population to local communities. If race and ethnicity are fundamental categories for biomedical analyses, they should be informative at all points along that continuum, much as the definition of a gene remains unchanged whether analyzed in the context of an individual, a group, or the total human population. By examining the utility of racial and ethnic categories at a local community level, we can better evaluate whether those categories are indeed fundamental units for analysis. Alternatively, these categories may be analytic proxies useful primarily for approximating aggregate biological and social features of large populations at national and transnational levels of analysis.