This contribution is dedicated to the direct modelling the non-monotonic course of the separation function of a hydrocyclone. To calculate the separation curve it is necessary to know the concentration field of particles of each size fraction in the apparatus. The system of equations describing the steady-state turbulent flow of a polydisperse suspension consisting of a liquid phase (water) and N fractions of the dispersed phase (particles) in the hydrocyclone, in line with the "mixtures model" can be written in a form of conservation equations. This system is investigated numerically. The main feature of these calculations is that previously developed model for the settling velocity of a polydisperse suspension in a centrifugal field is used. The results can be summarised as following: The consideration of the interaction of particles in a polydisperse suspension can explain the non-monotonic character of the separation curve (the Fish-Hook-effect) by the deviation of the particles' sedimentation rate from the law of Stokes. The magnitude of the Fish-Hook-effect depends on the concentration of the solid phase. (C) 2013 Elsevier Ltd. All rights reserved.