Fractionation of kraft wood pulp fibres was carried out in a hydrocyclone. Long fibres tend to migrate to the low shear region and report to the overflow stream. Excellent fractionation was found at high pulp slurry feed rates with a split ratio between 0.07 and 0.3. However, when air bubbles are sparged, all the fibres are more uniformly distributed inside the hydrocyclone. When increasingly more bubbles are present, on the other hand, long fibres drain down slower because of rapidly rising air bubbles. Thus, more long fibres report to the overflow stream at extremely high air injection rates.