An integrated mill producing kraft pulp (ca. 420,000 t a−1) and printing paper from hardwood and softwood was investigated for the ecotoxicological advantages due to complete substitution of chlorine by chlorine dioxide in the bleaching and replacement of aerated lagoons by an activated sludge plant in the treatment of effluents. A comparative study on Lake Saimaa (SE Finland) was performed before (1990/91) and after (1993) the process changes, and included the following sections: discharges of chlorinated organics and selected wood extractives from the mill, distribution and persistence of xenobiotic chemicals in the lake, their association with sedimenting particles and bioavailability, and biomarker responses in caged whitefish and feral fish (roach, bream, perch) exposed in waters polluted by bleached kraft mill effluent. The data revealed dramatic reductions in organochlorines, like chlorinated phenolics, both in the lake and fish. Based on the responses in fish, the novel environmental technology in the mill was reflected as much less exposure and effects in the aquatic biota. As a consequence, the lake area affected toxicologically was substantially reduced. Laboratory data on exposed whitefish supported the conclusions drawn from the field observations.