Atlantic salmon, Salmo salar, farms face the ongoing challenge of managing sea lice Lepeophtheirus salmonis and Caligus elongatus , infestations due to reduced sensitivity to extensively used chemotherapeutants. Imidacloprid is a neonicotinoid insecticide that has been implemented as a novel chemotherapeutant to mitigate sea lice infestations on European salmonid farms. Prior to its consideration for use in the Northwest Atlantic, we tested the effects of imidacloprid waste-water and dispersal concentrations on the ecologically and commercially valuable nontarget species, the American lobster, Homarus americanus . Behavior and hemolymph biochemistry of subadult female lobsters were assessed following a 120-minute exposure to the imidacloprid concentrations 0, 0.3 or 30 µg/L. Observations were repeated after a five-day recovery period to evaluate chronic effects. Defensive behaviors were found to be significantly reduced for lobsters from the 0.3 and 30 µg/L imidacloprid concentration groups compared to the control group, 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 low environmentally relevant concentrations of imidacloprid could impair lobster behaviors of ecological and economic importance.