Di- and trifluorosubstituted alkene compounds of formula I ##STR00001## wherein X is hydrogen or fluorine; Y is oxygen, NR.sup.1 or S(O).sub.m; R.sup.1 is hydrogen or C.sub.1 C.sub.6-alkyl; m is 0, 1, or 2; A,B,D and E are selected from the following: a) A is N and B, D and E are CR.sup.2; or b) B is N and A, D and E are CR.sup.2; or c) D is N and A, B, and E are CR.sup.2; or d) A and D are N and B and E are CR.sup.2; or e) B and E are N and A and D are CR.sup.2; R.sup.2 is H, halogen, NH.sub.2, NO.sub.2, CN, alkyl, haloalkyl, alkenyl, alkoxy, haloalkoxy, alkylthio, haloalkylthio, alkylsulfinyl, haloalkylsulfinyl, alkylsulfonyl, haloalkylsulfonyl, aminosulfonyl, alkoxyalkyl, alkylthioalkyl, alkylsulfinylalkyl, alkylsulfonylalkyl,alkylaminoalkyl, dialkylaminoalkyl, hydroxycarbonyl, or alkoxycarbonyl; or phenyl which may be substituted with halogen, CN, NO.sub.2, alkyl, haloalkyl, alkoxy, or haloalkoxy; or a 5- to 6-membered heteroaromatic ring system containing 1 to 3 heteroatoms selected from oxygen, sulfur and nitrogen, which may be substituted with halogen, CN, NO.sub.2, alkyl, haloalkyl, alkoxy, or haloalkoxy; n is 1, 2, 3 or 4, and the agriculturally and/or physiologically tolerable salts thereof, methods for the preparation of compounds I, and compositions and methods for the control of nematodes and arachnids, and for treating, controlling, preventing and protecting warm-blooded animals, fish and humans against infestation and infection by helminths, arachnids and arthropod endo- and ectoparasites.