The aim of the work was to study treatment efficiency and flux of various, new ceramic membranes in the ultrafiltration of board mill wastewater fractions. The wastewater fractions studied were the disk filtered and flotation treated circulation wastewater and the overflow from the clarification basin of coating colour wastewater. The membranes used were flat sheet alpha- and gamma-alumina membranes and modified alpha-alumina membranes. The modifications were made in order to improve flux and to decrease the fouling tendency of the membranes. The stabilized fluxes achieved after 1 h of filtration were in the range of 80-90 l/m(2)h with the alpha-alumina membranes, about 60 l/m(2)h with the gamma-alumina membranes and about 100-160 l/m(2)h with the modified alumina membranes. The treated circulation wastewater fouled the membranes more than the treated coating colour wastewater. Ultrafiltration removed the suspended solids and colloidal material from the water. Both wastewater fractions studied had a fairly low CODCr content and hence the reductions in CODCr were low. Iron was removed from the treated coating colour wastewater and odour from the treated circulation wastewater. Removal of odour and taste forming properties is important since the box board manufactured in the board mill is used for food, cosmetics and cigarette packings.