Slow O-demethylation of methyl triclosan to triclosan, which is rapidly glucuronidated and sulfonated in channel catfish liver and intestine

► This is the first study of triclosan and methyl triclosan biotransformation in a fish species ► Methyl triclosan was very slowly O-demethylated in the channel catfish liver and intestine ► The demethylation was inhibited by DMSO, a solvent commonly used in toxicology studies ► Triclosan was rapidly conjugated by glucuronidation and sulfonation ► Our results suggest methyl triclosan, but not triclosan, will bioaccumulate in exposed catfish.