There are disclosed novel avermectin derivatives wherein the outer oleandrose ring of avermectin or avermectin monosaccharide is homologated by addition of diazomethane to 4"- or 4'-oxo-avermectin to afford the perhydrooxepine derivatives. The homologated avermectins can then be further derivatized to afford additional novel avermectins. The new compounds are potent antiparasitic agents, in particular, the compounds are anthelmintic, insecticidal and acaricidal agents.