A life cycle experiment with fathead minnows and Ceriodaphnia survival reproduction tests were used to evaluate the quality of the effluent from a bleached kraft mill after the implementation of various process modifications and effluent treatment changes In the life cycle experiment the fish were exposed in the laboratory to well water control and five concentrations 1 25 2 5 5 10 or 20 of effluent from the egg stage to sexual maturity and reproduction 190 d None of the effluent concentrations signifi cantly affected the hatching of the eggs the mortality weight length gender balance reproduction and prevalence of visible morphologicalor histopathologicalabnormalities of the hatched fish and the hatchability of the first generation eggs In Ceriodaphnia tests the IC25 of the effluent affecting reproduction was approximately 80 This threshold concentration is well above the 0 7 average yearly concentration of the effluent that exists in the recipient near the point of discharge The results of these biotests were com pared to the results of the same biotests conducted earlier with the effluent from the mill prior to process and treatment modifications The comparison indicated that since the ear lier work the quality of the mills effluent improved substantially Threshold concentrations affecting fathead minnows in the life cycle experiment and the Ceriodaphnia tests increased by more than eightfold and approximately twofold respectively While the most important change in the mill operating conditions responsible for the improvement could not be identified these results indicate that mills can undertake process and treatment modifications that result in the discharge of effluents seemingly compatible with the aquatic environment