epi-Ivermectin B1a is a base-catalysed intermediate in the decomposition of ivermectin. epi-Ivermectin B1a is formed by epimerisation at the 2-position which ultimately rearranges irreversibly to the isomeric alkene analogue, Δ2-ivermectin B1a. Despite the importance of ivermectin as an anthelmintic in animal health, there are no published reports of the biological activity or the levels of epi-ivermectin B1a in animals or the environment.