There are disclosed novel avermectin compounds wherein the glycosyl side chain is substituted with, or replaced by an alkyl ester group in a Reformatsky reaction. The compounds are prepared by reacting an .alpha.-halogenated ester to a 4'-, 4"- or 13-keto-avermectin derivative in the presence of a reducing agent. The avermectin compounds have utility as anti-parasitic agents and compositions for that use are also disclosed. The compounds are also highly potent insecticides against agricultural pests.