The mucosal lymphocyte integrin α E (CD103) β 7 is thought to be important for intraepithelial lymphocyte (IEL) localization or function. We cloned the murine integrin gene encoding α E, localized it to chromosome 11, and generated integrin α E-deficient mice. In α E−/− mice, intestinal and vaginal IEL numbers were reduced, consistent with the known binding of α E β 7 to E-cadherin expressed on epithelial cells. However, it was surprising that lamina propria T lymphocyte numbers were diminished, as E-cadherin is not expressed in the lamina propria. In contrast, peribronchial, intrapulmonary, Peyer’s patch, and splenic T lymphocyte numbers were not reduced in α E-deficient mice. Thus, α E β 7 was important for generating or maintaining the gut and vaginal T lymphocytes located diffusely within the epithelium or lamina propria but not for generating the gut-associated organized lymphoid tissues. Finally, the impact of α E deficiency upon intestinal IEL numbers was greater at 3–4 wk of life than in younger animals, and affected the TCR αβ+ CD8+ T cells more than the γδ T cells or the TCR αβ+ CD4+ CD8− population. These findings suggest that α E β 7 is involved in the expansion/recruitment of TCR αβ+ CD8+ IEL following microbial colonization. Integrin α E-deficient mice will provide an important tool for studying the role of α E β 7 and of α E β 7-expressing mucosal T lymphocytes in vivo.