To compare vulva development mechanisms in the nematode Oscheius sp. 1 to those known in Caenorhabditis elegans, we performed a genetic screen for vulva mutants in Oscheius sp. 1 CEW1. Here we present one large category of mutations that we call cov, which affect the specification of the Pn.p ventral epidermal cells along the antero-posterior axis. The Pn.p cells are numbered from 1 to 12 from anterior to posterior. In wild-type Oscheius sp. 1 CEW1, the P(4-8).p cells are competent to form the vulva and the progeny of P(5-7).p actually form the vulva, with the descendants of P6.p adopting a central vulval fate. Among the 17 mutations (defining 13 genes) that we characterize here, group 1 mutations completely or partially abolish P(4-8).p competence, and this correlates with early fusion of the Pn.p cells to the epidermal syncytium. In this group, we found a putative null mutation in the lin-39 HOM-C homolog, the associated phenotype of which could be weakly mimicked by injection of a morpholino against Osp1-lin-39 in the mother's germ line. Using cell ablation in a partially penetrant competence mutant, we show that vulval competence is partially controlled by a gonadal signal. Most other mutants found in the screen display phenotypes unknown in C. elegans. Group 2 mutants show a partial penetrance of Pn.p competence loss and an abnormal centering of the vulva on P5.p, suggesting that these two processes are coregulated by the same pathway in Oscheius sp. 1. Group 3 mutants display an enlarged competence group that includes P3.p, thus demonstrating the existence of a specific mechanism inhibiting P3.p competence. Group 4 mutants display an abnormal centering of the vulval pattern on P7.p and suggest that a specific mechanism centers the vulval pattern on a single Pn.p cell.