The objective of this project was to measure the response of a common northern river invertebrate (mayfly) to the combined effects of pulp mill effluent and low winter DO [dissolved oxygen] levels. Experiments were conducted under laboratory conditions using artificial streams and the combined bleached kraft pulp mill and municipal sewage effluent from Hinton. Mayflies were exposed to two different concentrations of effluent (control river water and 1% effluent) and two levels of DO (5 mg/L and 11 mg/L). The measured responses of mayflies included feeding rate, survival, and positioning of individuals within the streams.