A sensitive, accurate and reproducible assay for the determination of residual concentrations of ivermectin in the tissues of farmed Atlantic salmon is described. The assay employs the clean‐up of an acetonitrile extract of salmon tissue on a Bond‐Elut C8column, followed by derivatization with trifluoroacetic acid anhydride. Following separation using HPLC, 22,23‐dihydroavermectin B1ais determined using fluorescence detection. The limit of detection of the assay is 1 ng per g tissue. Liver, taken from fish treated experimentally with ivermectin, was found to contain higher concentrations of 22,23‐dihydroavermectin B1athan any other tissue examined. The half‐life of 22,23‐dihydroavermectin B1awas similar in all tissues studied, ranging from 89 to 98 degree days.