Zebra (Dreissena polymorpha Pallas, 1771) and quagga (Dreissena rostriformis bugensis Andrusov, 1897) mussels are important pests in North America and Europe, where they cause costly economic damage to water infrastructure and severe environmental impacts to lake ecosystems. Strategies for combating these invasive mussels do not differentiate between the species, and little is known about how they may differ in terms of their susceptibility to chemical control methods. In this study, adult zebra and quagga mussels were collected from Lake Ontario and exposed to three different concentrations of EarthTec QZ – 60, 120, and 180 μg/L as liquid ionic copper – until 100% mortality was reached. Quagga mussels died significantly faster and at lower doses of copper ion than zebra mussels. Dry tissue collected from the mollusks at the time of death was analyzed for the copper concentration in each species and results revealed that zebra mussels accumulated and were able to tolerate much higher concentrations of copper in their tissues than quagga mussels. After 10 days’ exposure to the highest dose rate (180 μg/L) zebra mussels had experienced less than 20% mortality while accumulating more than 1,000 mg/kg copper, whereas the quagga mussels treated at the same 180 ug/L suffered 100% mortality after 10 days despite having accumulated only 300 mg/kg of copper. While more research is needed to explain the precise mechanisms whereby zebra mussels are able to tolerate the much higher bodily concentrations of copper than quagga mussels, these results have direct management implications when copper is used for dreissenid control.