The chronic effects of secondary‐treated effluent from a thermomechanical pulp (TMP) mill were assessed by means of long‐term and short‐term laboratory toxicity tests. The effluent used for the tests was sampled at a western Canadian mill using mostly softwoods and <10% recycled fiber as furnish. In the long‐term test, the effects of the effluent on the life cycle of fathead minnows (Pimephales promelas.) were studied. In this experiment, which began with the egg stage and continued through to sexual maturity and reproduction, the fish were exposed in the laboratory to well water (control) and five concentrations (1.25%, 2.5%, 5%, 10%, or 20%) of effluent for 202 d. None of the effluent concentrations significantly affected the hatching of the eggs, the mortality, weight, length, gonad size, gender balance, and reproduction of the hatched fish, the prevalence of gross morphological and histopathological changes, and the hatchability of the first generation eggs. Two short‐term tests, each lasting 7 d, were also performed. In these tests, 100% effluent caused no change in the survival/growth of minnow larvae or in the survival/reproduction of Ceriodaphnia.