Epi-Doramectin is a base-catalysed intermediate in the decomposition of doramectin. epi-Doramectin is formed by epimerisation at the 2-position which ultimately rearranges irreversibly to the isomeric alkene analogue, ?2-doramectin. Despite the importance of doramectin as an anthelmintic in animal health, there are no published reports of the biological activity or the levels of epi-doramectin in animals or the environment.