Abstract A study was conducted to determine the enzymatic kinetic parameters V max , K M , and intrinsic clearance (CL int ) for the hepatic in vitro production of aflatoxin B 1 -dihydrodiol (AFB 1 -dhd) from aflatoxin B 1 (AFB 1 ) in four commercial poultry species, ranging in sensitivity to AFB 1 from highest (ducks) to lowest (chickens). Significant but small differences were seen for V max , while large significant differences were observed for K M . However, the largest inter-species differences were observed for the CL int parameter, with ducks being extraordinarily efficient in converting AFB 1 into AFB 1 -dhd. Since AFB 1 -dhd is considered the metabolite responsible for the acute toxic effects of AFB 1 , the high hepatic production of AFB 1 -dhd from AFB 1 in ducks is the possible biochemical explanation for the extraordinary high sensitivity of this poultry species to the adverse effects of AFB 1 .