The aims of this study were to investigate the content of emamectin in blood, mucus and muscle following field administration of the recommended dose, and correlation with sea lice infection on the same fish (elimination study). The tissue distribution of tritiated emamectin benzoate after a single oral dose in Atlantic salmon was also investigated by means of whole-body autoradiography and scintillation counting (distribution study). In the elimination study, concentrations of emamectin benzoate reached maximum levels of 128, 105 and 68 ng/g (p.p.b.) for blood, mucus and muscle respectively, on day 7, the last day of administration. From day 7, the concentration in the blood declined until concentration was less than the limit of detection on day 77. The concentration was higher in mucus compared with plasma (P < 0.05) except on days 7 and 21. The concentration of emamectin benzoate decreased gradually from the end of treatment (day 7) to day 70 with half-lives of 9.2, 10.0 and 11.3 days in muscle, plasma and mucus respectively. The distribution study demonstrated a high quantity of radioactivity in mucous membranes (gastrointestinal tract, gills) throughout the observation period (56 days). Activity was high in the epiphysis, hypophysis and olfactory rosette throughout the study. The highest activity was observed in the bile, indicating this to be an important route for excretion. The distribution study confirmed the results from the elimination study with respect to concentrations in blood, skin mucous and muscle.