Avermectin compounds are substituted at the 4", 4' or 13-postion hydroxy group with an alkylthioalkyl group and are optionally substituted at the other reactive positions of the avermectin molecule. The compounds are prepared by reacting protected avermectins, avermectin monosaccharides or avermectin aglycones with dialkylsulfoxides. The compounds are potent antiparasitic agents and compositions for such uses are also disclosed.