The increased environmental awareness and stringent legislation are forcing the pulp and paper mills to reduce their water consumption. This can be achieved by recycling the waters both internally and externally. Membrane technologies offer one possibility to further improve the quality of the wastewater. In this study, a new type of ceramic membrane was used to ultrafilter a biologically treated combined board mill wastewater. The flat membrane was made of alpha -alumina and it was modified with titania. The mean pore size of the membranes was 100 nm. Backflushing was used as a means to improve the permeate flux. Earlier tests have shown that backflushing can improve the flux with 11-28% when done for 1 s every minute at a pressure of four bar from the permeate side. The decrease observed in retentions of major quality parameters (suspended solids, turbidity and CODCr) was only about 5%. The effect of different backflushing parameters on flux and retentions will be described in this paper. The parameters studied were backflushing frequency, length of the backflush, and the backflushing pressure. The results show that the best permeate flux was obtained,when backflushing was made for I s every 2 min at a pressure of four bar from the permeate side. From the parameters studied the length of the pulse was the most important factor affecting the permeate flux. The second most important factor was the backflushing frequency. The effect of backflushing pressure was not as obvious, but at least in these cases the decrease in backflushing pressure increased the permeate flux.