Abstract: The present invention relates to the use of substituted alkoxydiphenyl ethers or alkoxydiphenylmethanes of the general formula I ##STR1## in which R.sup.1 represents hydrogen, halogen, alkyl, alkoxy, alkylthio, halogenoalkyl, halogenoalkoxy, halogenoalkylthio, dioxalkylene, dioxyhalogenoalkylene, CN, NO.sub.2, alkenyl, alkinyl, alkoxyalkyl, alkoxyalkoxy or hydroxyalkoxy,R.sup.2 represents the radicals indicated for R.sup.1,R.sup.3 represents the radicals indicated for R.sup.1,R.sup.4 represents hydrogen, alkyl, halogenoalkyl or halogen,R.sup.5 represents the radicals indicated for R.sup.4,Het represents optionally substituted heteroaryl which is not linked to the remaining radical via the hetero atom,X, Y independently of one another represent --O-- or --S--Z represents --O--, --S--, --CH.sub.2 --, --CHCH.sub.3 -- or --C(CH.sub.3).sub.2 --,m and n independently of one another represent 0, 1, 2 or 3 but their total is equal to, or more than, 2are exceedingly suitable for controlling fleas.