Liver microsomes from market‐size ( n = 6) rainbow trout, channel catfish and tilapia were used to investigate in vitro biotransformation kinetics of albendazole (ABZ). ABZ was transformed to a single metabolite, ABZ sulfoxide (ABZ‐SO). Catfish displayed the highest maximal velocity ( V max = 264.0 ± 58.6 pmols ABZ‐SO/min/mg protein) followed by tilapia (112.3 ± 8.2) and rainbow trout (73.3 ± 10.3). V max in catfish was significantly different ( P < 0.05) from the other two species. Michaelis–Menten constant ( K m ) values (μ m ) varied significantly among the species: rainbow trout (3.9 ± 0.5), tilapia (9.2 ± 1.7) and catfish (22.0 ± 3.2). However, V max / K m ratios showed no difference among the three species, making them equally efficient performing this phase I biotransformation reaction. In a second series of experiments, channel catfish ( n = 6 per treatment) were dosed in vivo with gel‐food containing ABZ (10 mg/kg, p.o.). Fish were killed at 24, 48, 72 and 120 h after dosage. Control fish were fed ABZ‐free feed. Induction of ethoxyresorufin‐o‐deethylase activity was significant ( P < 0.05) in all ABZ‐dosed treatments as compared with controls.