PURPOSE: To provide a new compd. having excellent long-term duration and useful as an antiparasitic agent and anthelmintic.

CONSTITUTION: This compd. is expressed by formula I, wherein R is sec-butyl or isopropyl, and the compd. is, for example, 13β-O-methoxymethyl-22,23- dihydroavermectin BIa/BIb aglycone. The compd. of formula I is obtd. by allowing an avermectin aglycone expressed by formula II to react with a halomethyl ether expressed by CH3OCH2Cl, especially chloromethyl methyl ether in the presence of a base (e.g. triethylamine) in an anhydrous aprotic solvent (e.g. methylene chloride) at 10 to 120°C for 12 to 24 hours. The avermectin is obtd. by isolating from a fermentation soln. of Streptomyces avermitilis or its deriv.