In Caenorhabditis, the vulva is formed in the central body region from three of six equivalent cells and is induced by the gonad. In some nematodes, however, the vulva is located in the posterior body region. Vulval development has been analyzed in three such genera. The same precursor cells give rise to the vulva in Caenorhabditis and in the posterior vulva species, but in the latter the cells first migrate posteriorly. In two such species, the vulva is not induced by the gonad, but instead relies on intrinsic properties of precursor cells. Thus, evolution of organ position involves changes in induction and competence.