PURPOSE: To provide a new compd. useful as an antiparasitic agent and a reagent against enterozoon.

CONSTITUTION: This compd. is expressed by formula I, wherein R is -CO-R9, -CH(OH)-R9, a 3 to 8C cycloalkyl, phenyl; R1 is H, a 1 to 10C alkyl, 3 to 8C cycloalkyl, phenyl, oxygen- or sulfur-contg. 3 to 6-member heteroring, etc.; R2 is OH, a 1 to 10C alkoxy, etc.; R3 is H, OH, a 1 to 10C alkoxy, etc.; R9 is a 1 to 8C alkyl, 1 to 8C alkoxy, phenyl, etc. The compd. is, for example, 4",5-di-O-t-butyldimethylsilyl-7-O-tritylsilyl-21,25-seco-24-desmethyl-25-des(2- butyl)-21-methoxy-25-nor-24-oxo-avermectyl. The compd. of formula I is obtd. by cyclizing a compd. expressed by formula II in the presence of a mixture catalyst of pyridine and P-toluenesulfonic acid. The obtd. compd. is effective for parasites, especially, helminth, exoteric parasites, insects and sarcoptids.