Zebra mussels (Dreissena polymorpha Pallas 1771) are a tenacious aquatic invasive species in the United States and new infestations can rapidly expand into dense colonies. Zebra mussels were first reported in Marion Lake (Dakota County, Minnesota) in September 2017 and surveys indicated the infestation was likely isolated near the public boat access. A 2.4-hectare area, containing the known zebra mussel infestation, was enclosed and treated for nine days with EarthTec QZ®, a copper-based molluscicide, in an attempt to eradicate the zebra mussels. We conducted an on-site bioassay to confirm treatment efficacy. The bioassay was conducted in mobile assay trailer that received a continuous flow of treated lake water and zebra mussels test animals that were collected from White Bear Lake (Ramsey County, MN). Zebra mussel mortality in the treated bioassay tanks achieved 99 percent mortality (95-percent Confidence Interval: 98 – 100 percent) and a predictive model indicates in-lake mortality as low as 85 percent may have occurred; therefore, our results indicate that the Marion Lake treatment may not have induced complete zebra mussel mortality. Post-treatment assessments and surveys are recommended to further verify treatment success and monitor lake conditions in regards to zebra mussel infestation.