The pesticide formulation Salmosan (47.5% w/w azamethiphos) is currently registered for use, in Canada, to treat salmonids for infestations of the copepod parasites, Lepeophtheirus salmonis and Caligus elongatus (sea lice). Determination was made of the acute lethality of this product to the three larval stages, the first postlarval stage, and the adult of the American lobster (Homarus americanus), a species of significant economic importance in Eastern Canada. The 48-h LC50 (as azamethiphos) is 3.57 microg/liter for Stage I, 1.03 microg/liter for Stage II, 2.29 microg/liter for Stage III, 2.12 microg/liter for Stage IV (the first postlarval stage), and 1.39 microg/liter for adults. These concentrations are not significantly different from each other, although the variability in response is greater in the larval stages than in the postlarvae or adults. These data when interpreted in conjunction with known physical oceanographic data and chemical dispersion studies indicate that single anti-louse treatments are unlikely to result in mortality among lobsters in the vicinity of salmon farms. However, the sublethal effects of this product and the effects of repeated exposures have yet to be determined.