Avermectin analogs are disclosed wherein the 6,6-spiroketal ring system has been reduced in size to a 6,5-spiroketal ring system by the deletion of the 25-position carbon atom and the 25-alkyl substituent. This is accomplished by opening the outer spiroketal ring with the elimination of ring carbon atoms 23, 24 and 25 and the alkyl substituent at the 25-position and incorporation a new component, reclosing the spiroketal to a 5-membered ring with new substituents at the 24-position. The compounds are used as anti-parasitic insecticidal and anti-helmintic agents in humans and animals and compositions containing such compounds as the active ingredient thereof are also disclosed.