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T regulatory cells that express the transcription factor Foxp3 (Foxp3(+) T(regs)) promote tissue homeostasis in several settings. We now report that symbiotic members of the human gut microbiota induce a distinct T(reg) population in the mouse colon, which constrains immuno-inflammatory responses. This induction—which we find to map to a broad, but specific, array of individual bacterial species—requires the transcription factor Rorγ, paradoxically, in that Rorγ is thought to antagonize FoxP3 and to promote T helper 17 (T(H)17) cell differentiation. Rorγ's transcriptional footprint differs in colonic T(regs) and T(H)17 cells and controls important effector molecules. Rorγ, and the T(regs) that express it, contribute substantially to regulating colonic T(H)1/T(H)17 inflammation. Thus, the marked context-specificity of Rorγ results in very different outcomes even in closely related cell types.