Rotor blades in axial flow compressors and turbines in gas turbine engines commonly have firtree roots retained in correspondingly shaped slots in a rim of a disc. The blades typically have integral platforms which butt together when the blades are assembled on the disc to define a cylindrical inner wall of an annular gas flow path. Stresses induced by high rotor speeds concentrate at the firtree slots and may be minimized by minimizing the mass of the blades. To that end, rotors have been proposed wherein the blades include only airfoils and roots, the platforms being separately attached structural elements. In one proposal, individual platforms are hinged to the disc between the airfoils. In another proposal, the platforms are inserts which fit around the airfoils and are retained by hooked portions which lodge in the slots at opposite ends of the blade roots. In still another proposal, individual T-shaped platforms are disposed between the airfoils and retained in slots in the disc between the blade retention slots. And in yet another proposal, individual platforms between the airfoils have wedge shaped ends which fit into the blade retention slots along side the blade roots. In a related proposal for a light-weight rotor, a pair of annular side plates on a shaft are welded together on opposite sides of discs from which sheet metal blades are formed, the blades projecting radially out through slots in a rim formed by the welded-together end plates. A turbomachine rotor according to this invention has a platform separate from the rotor blades which is simple to assemble on the rim of the rotor disc and which is attached to the rim remote from the most highly stressed regions thereof.