Overgrowing of filamentous micro-organisms in activated sludge is affected by the following factors: (a) composition of treated waste water; (b) actual concentration of dissolved oxygen in an aeration tank; (c) actual concentration of soluble substrate under which micro-organisms grow; (d) technological parameters of the process (sludge loading and age). All these factors were analysed and discussed in the light of the formulated basic principles for control of filamentous bulking.