Atlantic salmon farms face the ongoing challenge of managing sea lice infestations due to reduced sensitivity to extensively used chemotherapeutants. Imidacloprid is a neonicotinoid insecticide that has been implemented as a novel chemical to mitigate sea lice infestations on European Atlantic salmon farms. Prior to its consideration for use in the Northwest Atlantic , we tested the effects of these imidacloprid waste-water and dispersal concentrations on the economically valuable nontarget species, the American lobster , Homarus americanus. Behavior and hemolymph biochemistry of sub-adult female lobsters were observed following 120-minute exposure to the imidacloprid concentrations 0, 0.3 or 30 ppb. Observations were repeated five days later to evaluate chronic effects. Defensive behaviors were found to be significantly reduced for lobsters exposed to the 30 ppb imidacloprid concentration, and some remained significantly reduced five days after exposure. Interestingly, overall hemolymph endpoints indicative of stress (L-lactate, crustacean hyperglycemic hormone, and total protein) were not significantly different across treatments. This highlights the importance of behavioral endpoint measurement, as limited upstream endpoints may not always capture the full impact downstream. These findings suggest that if salmon farms of the North west Atlantic administer imidacloprid as a treatment for sea lice, nearby lobsters may have impaired behaviors of ecological and economic importance.