Recent concerns have focused on the adverse effects of low dissolved oxygen (DO) combined with the potential chronic effects of pulp and paper mill effluent exposure on fishes in the Tarawera River, Bay of Plenty, New Zealand. A degassing system for the removal and control of dissolved oxygen (DO) capable of producing water with-0.5 mg L⁻¹ DO to saturation and two series of five separate DO concentrations each was first constructed. Because the Tarawera River receives effluent discharges from a chemithermomechanical (CTMP) tissue mill and an integrated thermomechanical (TMP)/bleached kraft pulp and paper mill (BK), these effluents were studied. Using the DO control system, the effects of simultaneous effluent and hypoxia exposure on the 48-h DO LC50 (median lethal concentration) were examined in fry and juvenile rainbow trout (Oncorhynchus mykiss) and common bully (Go_biomorphus cotidianus). TMPIBK mill effluent (ME) was diluted to 15 % v/v to reflect the upper concentration in the receiving environment, while CMTP effluent was extracted directly from the river. The presence of either effluent did not significantly increase the lethality of low DO for either species and life stage differences were not detected. Preexposure of juvenile trout to 15 and 70 % TMP/BKME also showed no significant effect on the DO LC50. No significant difference in the oxygen consumption of juvenile trout tested in 15 % v/v effluent were evident, but significant increases in oxygen consumption were observed for fish pre-exposed to 15 and 70 % effluent when tested in reference water. Four-week exposures were performed with juvenile trout to determine the chronic effects ofTMP/BKME (15 % v/v) and CTMP effluent combined with non-lethal DO concentrations (2.5 mg L⁻¹ to fully saturated), revealing excellent survival of fish in the combined effluent/hypoxia exposures, but marred by poor survival of reference water-exposed fish. Relationships for growth were observed for fish exposed to DO and effluent in the CTMP experiment. Several DO dose-response effects were also observed on general hematology in both effluent experiments, but were generally considered minor and typical of hypoxia exposure in fish. Four-week exposures using juvenile trout in 0, 10, 30 and 70 % TMP/BKME and 0, 35, 110 and 250 μg L⁻¹dehydroabietic acid (DHAA) were performed, where swimming performance, oxygen consumption and hematology were investigated. The chief finding was that of reduced swimming performance, indicated by lower critical swimming (Ucrit) speeds, for fish exposed to 70 % TMP/BKME. Other moderate effects on various hematological parameters were also observed. However, all observed effects are considered relatively small in magnitude and at effluent concentrations well above those found in the receiving environment. Forty eight-hour DO LCSOs were determined for juvenile inanga (Galaxias maculatus), common smelt (Retropinna retropinna), shortfin eel elvers (Anguilla australis), rainbow trout, common bully and the freshwater shrimp (Paratya curvirostris). Juvenile inanga were the most sensitive fish species, while eel elvers were the most tolerant fish species tested.