Abstract: In a rotor construction for hammer- or chopper mills the rotor is formed by a cylindrical shell structure (2) transversally profiled for providing a drum surface comprising at least one longitudinally extending recess (6) for accommodation of base portions of chopper members. In the rotor construction the at least one recess (6) is provided with chopper positioning means (8, 18) at fixed locations for lateral stabilisation of said base portions and for stable accommodation of shaft members (12) for pivotable attachment of said chopper members (14).