Lymphoid tissue-inducer (LTi) cells initiate the development of lymphoid tissues through the activation of local stromal cells in a process similar to inflammation. LTi cells express the nuclear hormone receptor ROR gamma t, which also directs the expression of the proinflammatory cytokine interleukin-17 in T cells. We show here that LTi cells are part of a larger family of proinflammatory ROR gamma t(+) innate lymphoid cells (ILCs) that differentiate from distinct fetal liver ROR gamma t(+) precursors. The fate of ROR gamma t(+) ILCs is determined by mouse age, and after birth, favors the generation of cells involved in intestinal homeostasis and defense. Contrary to ROR gamma t(+) T cells, however, ROR gamma t(+) ILCs develop in the absence of microbiota. Our study indicates that ROR gamma t(+) ILCs evolve to preempt intestinal colonization by microbial symbionts.