Abstract. Zebra mussels (Dreissena polymorpha, Pallas, 1771) are an aquatic invasive species in the United States, and new infestations of zebra mussels 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 a public boat access. A 2.4-hectare area containing the known zebra mussel infestation was enclosed and treated by area resource managers for 9 days with EarthTec QZ (target concentration: 0.5 milligrams per liter as copper), a copper-based molluscicide, to eradicate the zebra mussels. Researchers led an onsite bioassay to provide an estimate of the treatment efficacy within the enclosure. The bioassay was conducted in a mobile assay trailer that received a continuous flow of treated lake water. Bioassay tanks (n=9; 350 liters) within the trailer were stocked with zebra mussels (25 mussels per containment bag; 7 bags per tank) collected from White Bear Lake, Ramsey County, Minn. Mortality in the treated bioassay tanks reached a mean of 99 percent (95-percent confidence interval: 98–100 percent), there were no mortalities in the control tanks. However, a predictive model produced for timely delivery to area resource managers indicated zebra mussel mortality within the treated enclosure may have been as low as 85 percent. Onsite bioassays are a viable and important tool for treatment evaluation particularly in newly infested waterbodies with low zebra mussel densities.