An apparatus for stretching natural staple fibers to reduce their diameter includes a plurality of rotatable arrays of driven pulleys which are mounted for rotation between two twist blocking nips. The rotatable arrays are rotatable on a longitudinal axis corresponding to the direction of travel of an assembly of fibers through the apparatus to impart false twist into the assembly. The assembly of fibers is stretched between two of the pulley arrays with the pulleys of the downstream array being driven at a higher speed than the pulleys of the upstream array. A steam chamber is provided for setting the stretch while the false twist is being maintained by further rotatable pulley arrays. Post treatment stages may be added to further stabilize the stretched fibers.