Using 1990 census data, the authors compare 77 immigrant and ethnic groups in the 16 largest metropolitan regions in the United States. They find that the interaction effect of location and ethnicity on ethnic entrepreneurship is evident not only in self-employment rates but also in niche concentrations and niche competition. Their results reveal a distinction between mainstream groups and nonmainstream groups. Compared to mainstream groups, nonmainstream groups are more context resistant. That is, they concentrate in few entrepreneurial niches and display high niche continuity across metropolitan regions. Group competition influences niche concentrations, but an adverse impact on black entrepreneurship is not apparent.