A fan rotor including a disk carrying blades including roots engaged in slots in the disk, a frustoconical cap mounted on the disk upstream from the blades, and a retaining mechanism axially retaining the blades on the disk and including a ring mounted in an annular groove of the disk and forming an abutment for the roots of the blades, the ring being festooned or crenellated and co-operating with a festooned radial annular bead of the annular groove of the disk, and a mechanism preventing the ring from moving in rotation, which includes an annulus carrying at least one axial tooth inserted in hollow portions of the bead of the disk and of the ring, the annulus being fastened by bolts on an upstream radial face of the disk.