Abstract: A pressure sorter for fiber suspensions, and a process for the preparation of fiber suspensions, are disclosed wherein a rotor is disposed concentrically within a cylindrical screen that has spaced circumferential rows of narrow through slots through which a portion of the fiber suspension fed into the screen passes radially outward to an accepts chamber external to the screen. The fraction of the suspension which does not pass through the screening slots passes axially out of the screen cylinder as a "rejects" flow. V-shaped grooves around the inflow ends of the screening slots create turbulences in the adjacent suspension material. The rotor includes external profiled elements which generate positive and negative pressure pulses at the adjacent inflow side of the screen as the rotor is rotated.