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 assessed followin g a 120-minute exposure to the imidacloprid concentrations 0, 0.3 or 30 ppb. Observations were repeated after a five-day recovery period 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 observations, as limited upstream endpoints may not always capture the full impact downstream. These findings suggest that lobsters exposed to low environmental concentrations of imidaclopri d may have impaired behaviors of ecological and economic importance.