Avermectin monosaccharide is an acid degradation product produced by selective hydrolysis of the terminal saccharide unit of avermectin. Avermectin monosaccharide is a potent inhibitor of nematode larval development, but is devoid of paralytic activity. Despite the importance of avermectin as an anthelmintic in animal health, there are few published reports of the biological activity or the levels of avermectin monosaccharide in animals or in the environment.