An HPLC method for the determination of fenbendazole (FBZ) and its metabolites, fenbendazole sulphoxide (oxfendazole. FBZ-SO) and fenbendazole sulphone (FBZ-SO2) in trout and eel tissue is described. The compounds are extracted with ethyl acetate and the extract, after addition of hexane, is concentrated and cleaned up on a silica gel solid-phase extraction column. After elution with 3% acetic acid in methanol the reconstituted eluate is analysed on a Lichrosorb RP8 column, the mobile phase being 0.05 M ammonium phosphate (pH=5): acetonitrile:methanol (55:28:17,v/v). Detection is performed at 297 nm. The average recovery in trout muscle tissue over the concentration range 10–500 jig/kg is 75.7 ± 3.3 % and 77.5 ± 4.1 % for FBZ-SO and FBZ-SO2, respectively, and for FBZ the recovery is 76.3 ± 5.5 % over the range 15–750 μg/kg.