Schizosaccharomyces pombe defective in phosphatidylinositol (PtdIns) 3-kinase shows various defects in forespore membrane formation, including onset, growth orientation, and closure. Downstream factors of PtdIns 3-kinase in this system were explored. Among various phox homology (PX) domain-containing proteins, Vps5p and Vps17p, homologues of sorting nexins, were found to be required for efficient sporulation. Cells defective in these proteins showed a disordered growth orientation of the forespore membrane, as is the case with Deltapik3 cells. Vps5p and Vps17p with mutations in the PX domains failed to suppress the defects of their relevant disruptants. Vps5p and Vps17p migrated toward the the forespore membrane in a pik3+-dependent manner, suggesting that these proteins may interact with PtdIns(3)P. Electron-microscopic analysis revealed that the forespore membrane fails to engulf the nucleus in some of these cells, accumulating vesicle-like bodies similar to those seen in Deltaspo3 cells. These results suggest that Vps5p and Vps17p are the targets of PtdIns(3)P in vesicle transport required for onset of the forespore membrane formation.