Niclosamide produces genotoxic effects, such as point mutations in Salmonella sp., sperm-head abnormalities in mice and clastogenic effects in human lymphocytes in vitro and in vivo. As cytochrome P450 could be involved in the bioactivation of niclosamide, we investigated which subfamily was involved. We used liver microsomal fractions from rats treated with phenobarbital/β-naphthoflavone (PB/β-NF), benzo[a]pyrene (BaP) or cyclohexanol, which are known to induce different cytochrome P450 subfamilies, such as CYP2B, CYP1A1, CYP1A2 and CYP2E1. We also inhibited CYP1A and CYP2E using α-NF and diethyldithiocarbamate to identify the cytochrome P450 involved. Liver-S9 fractions obtained from PB/β-NF- and BaP-treated rats significantly increased the number of revertants induced by niclosamide, while the CYP1A1 inhibitor α-NF decreased the number of revertants. The incubation of niclosamide with CYP1A1 Supersomes™ increased the number of revertants, suggesting that CYP1A1 is responsible for the bioactivation of niclosamide. Nitroreduction is also involved in niclosamide bioactivation, as the nitroreductase-deficient strain YG7132 did not respond to the niclosamide treatment. Our findings indicated that a metabolite, derived from the action of CYP1A1 and a nitroreduction-reaction process, has a key role in the bioactivation of niclosamide.