Mammalian Ras proteins associate with multiple effectors, including Raf, Ral guanine nucleotide dissociation stimulator, phosphoinositide 3-kinase and AF-6. In the nematode Caenorhabditis elegans, LIN-45/Raf has been identified genetically as an effector of LET-60/Ras. To search for other effectors in C. elegans, we carried out a yeast two-hybrid screening for LET-60-associating proteins. The screening identified a novel protein, designated Ce-AF-6, which exhibited a strong structural homology with human AF-6, rat Afadin and Drosophila melanogaster Canoe and possessed both the Ras-associating (RA) domain and the PSD-95/DlgA/ZO-1 (PDZ) domain. Ce-AF-6 associated with human Ha-Ras in a GTP-dependent manner, with an efficiency comparable to that of human Raf-1 Ras-binding domain. When the effects of mutations of the Ras effector region residues were examined for associations with various effectors, Ce-AF-6 was found to possess a distinct and the most rigorous requirement for the effector region residues. These results strongly suggest that Ce-AF-6 is a putative effector of Ras that possesses a distinct recognition mechanism for association with Ras.