One of the best known features of vulva development in Caenorhabditis elegans is the induction of vulval precursor cells by the gonadal anchor cell. Induction is crucial for the initiation of pattern formation within the C. elegans vulva equivalence group, and it is therefore surprising to find that this aspect of vulva formation, in particular, varies greatly among nematodes. In some species which form vulvae in the posterior body region, no gonadal signal is necessary for vulva induction. In other nematodes, such as Panagrolaimus, Oscheius, and Rhabditella, vulva formation depends on two temporally distinct gonadal inductions which specify the different cell fates. Here we report our analysis of vulva induction in Pristionchus pacificus, a species which has recently been used as a genetic system to analyze the evolution of vulva development. Cell ablation studies in P. pacificus show that another mode of vulva induction exists. P. pacificus vulva formation depends on a continuous gonadal induction that starts several hours after hatching and continues until the birth of the anchor cell, some 20 h later. Mutations defective in gonadal induction result in the absence of vulva differentiation, suggesting that only one signaling system is involved in the gonadal-epidermal interaction. This new mode adds further to the great variety of gonadal inductions among nematode species.