A balance between pro- and anti-inflammatory mechanisms at mucosal interfaces, which are sites of constitutive exposure to microbes and non-microbial foreign substances, allows for efficient protection against pathogens yet prevents adverse inflammatory responses associated with allergy, asthma and intestinal inflammation(1). Regulatory T (T-reg) cells prevent systemic and tissue-specific autoimmunity and inflammatory lesions at mucosal interfaces. These cells are generated in the thymus (tT(reg) cells) and in the periphery (induced (i)T-reg cells), and their dual origin implies a division of labour between tT(reg) and iT(reg) cells in immune homeostasis. Here we show that a highly selective blockage in differentiation of iT(reg) cells in mice did not lead to unprovoked multi-organ autoimmunity, exacerbation of induced tissue-specific autoimmune pathology, or increased pro-inflammatory responses of T helper 1 (T(H)1) and T(H)17 cells. However, mice deficient in iT(reg) cells spontaneously developed pronounced T(H)2-type pathologies at mucosal sites-in the gastrointestinal tract and lungs-with hallmarks of allergic inflammation and asthma. Furthermore, iT(reg)-cell deficiency altered gut microbial communities. These results suggest that whereas T-reg cells generated in the thymus appear sufficient for control of systemic and tissue-specific autoimmunity, extrathymic differentiation of T-reg cells affects commensal microbiota composition and serves a distinct, essential function in restraint of allergic-type inflammation at mucosal interfaces.