Benomyl (a non-thio fungicide) inhibits hepatic mitochondrial low-K_m aldehyde dehydrogenase (mALDH or ALDH2) in ip-treated mice by 50% (IC₅₀) at 7.0 mg/kg, which is surprisingly the same potency range as that for several dithiocarbamate fungicides (and the related alcohol abuse drug disulfiram) and thiocarbamate herbicides previously known for their alcohol-sensitizing action. The mechanism by which benomyl inhibits mALDH was therefore examined, first by comparing the metabolism of benomyl with the aforementioned mono- and dithiocarbamates and second by evaluating the inhibitory potency of the benomyl metabolites. Benomyl in ip-treated mice is converted, via butyl isocyanate, S-(N-butylcarbamoyl)glutathione, and S-(N-butylcarbamoyl)cysteine, to S-methyl N-butylthiocarbamate (MBT), identified as a transient metabolite in liver. MBT is >10-fold more potent than benomyl or butyl isocyanate as an in vivo mALDH inhibitor and is also more potent than the intermediary S-(N-butylcarbamoyl) conjugates. Benomyl and MBT inhibit mouse hepatic mALDH in vitro with IC₅₀s of 0.77 and 8.7 μM, respectively. The potency of MBT is greatly enhanced by fortification of the mitochondria with NADPH alone or plus microsomes giving IC₅₀s of 0.50 and 0.23 μM, respectively. This activation of MBT is almost completely blocked by the cytochrome P450 inhibitor N-benzylimidazole but not by several other cytochrome P450 inactivators. MBT (probably following bioactivation) inhibits mALDH in vivo with an IC₅₀ of 0.3 mg/kg. Two candidate activation products were synthesized for potency determinations. N-Hydroxy MBT (prepared via the trimethylsilyl derivative) was not detected as an MBT metabolite; its low potency also rules against N-hydroxylation as the activation process. MBT sulfoxide, from oxidation of MBT with magnesium monoperoxyphthalate in water, is one of the most potent inhibitors known for mALDH and yeast ALDH in vitro (IC₅₀ 0.08−0.09 μM). These findings are consistent with a six-step bioactivation of benomyl, via the metabolites above and N-butylthiocarbamic acid, with MBT as the penultimate and MBT sulfoxide as the ultimate inhibitor of mALDH.