A repellant for repelling root-knot nematodes ( Meloidogyne spp .) that comprises as an active ingredient a substance secreted by a cellular slime mold belonging to the genus Dictyostelium. The substance secreted by the cellular slime mold can be obtained by, for example, culturing the cellular slime mold to form fruiting bodies, then separating the cellular slime mold and extracting. As the cellular slime mold, D. discoideum, D. purpureum, D. mucoroides, D. fasciculatum, D. monochasioides, D. lacteum or D. giganteum may be used. Thus, root-knot nematodes can be repelled without exerting any bad influence upon crops and workers.