Due to the limited chemotherapeutic options available, there is a continued reliance on emamectin benzoate (EMB) for the control of sea lice ( Lepeophtheirus salmonis) infestation on farmed Atlantic salmon ( Salmo salar), making resistance development a major concern. Previous studies indicate target site mutations and/or over-expression of P-glycoprotein (P-gp) to be the predominant mechanisms involved in the development of an avermectin resistance phenotype. This study primarily focused on the expression of P-gp in sea lice and its potential role in EMB resistance development. Two candidate genes, SL0525 and SL-Pgp1, were identified in sea lice from a sea lice expressed sequence tag (EST) database and RT-PCR on total RNA, respectively. We...