The magnitude and dynamics of oxygen depletion from zebra mussel metabolism for a severely infested 1.4 km section of the Seneca River, NY, is documented for a 3-month period of 1994, based on hourly measurements of dissolved oxygen (DO) concentrations at an upstream site and a downstream site and mass balance calculations. The average areal zebra mussel oxygen demand (ZOD) in the study section during a 30-day low-flow period (population density ≈40 000 individuals m^-2) was 39.4 g m^-2 d^-1. The average value of ZOD for the same 30-day period, normalized for zebra mussel biomass mass (42.4 mg of O₂ (g of zebra mussel)^-1 d^-1), compares favorably to recently reported oxygen consumption rates from laboratory experiments, sup porting its application elsewhere to accommodate the effects on oxygen resources of invaded systems. Values of ZOD varied greatly during the study reflecting responses of the invader to changes in ambient conditions, including flow and a major decrease in the population.