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Abstract. Anecdotal reports from the Scottish fish farming industry suggested a reduction in the sensitivity of salmon lice, Lepeophtheirus salmonis, to the organophosphate dichlorvos. In this study, pro-adult and adult lice were collected from marine salmon sites showing a variety of ‘sensitivities’ to dichlorvos and subjected to dichlorvos exposure at a range of doses for a 48-h period in vitro. From the lice mortalities, 24 and 48 h LC50 values were calculated, and using a ‘sensitive’ source of lice as a reference, resistance ratios calculated. The data generated suggests that L. salmonis from different geographic areas shows differences in sensitivity to dichlorvos. The implications for the salmon farming industry are discussed.