Antimicrobial peptides are important effectors of innate immunity throughout the plant and animal kingdoms. In the mammalian small intestine, Paneth cell α-defensins are antimicrobial peptides that contribute to host defense against enteric pathogens. To determine if α-defensins also govern intestinal microbial ecology, we analyzed the intestinal microbiota of mice expressing a human α-defensin gene (DEFA5) and in mice lacking an enzyme required for the processing of mouse α-defensins. In these complementary models, we detected significant α-defensin-dependent changes in microbiota composition, but not in total bacterial numbers. Furthermore, DEFA5-expressing mice had striking losses of segmented filamentous bacteria and fewer interleukin 17 (IL-17)-producing lamina propria T cells. Our data ascribe a new homeostatic role to α-defensins in regulating the makeup of the commensal microbiota.