Rotor blades having roots of the hammer-head type which are received in a circumferential groove in the rim of a rotor disc to secure the blades on the disc are provided with a raised portion on the underside of the blade platform which extends over the entire circumferential length of the platform and has a width equal to that of the mouth of the groove in the rim of the disc. The root of each blade is mounted on this raised portion, and when the root is received in the groove of the disc the raised portion is received closely in the mouth of the groove to ensure the correct alignment of the blade.