The present invention relates to novel trifluoromethyloxadiazoles of the formula I, or an N-oxide and/or their agriculturally useful salts and to their use for controlling phytopathogenic fungi; to a method for combating phytopathogenic harmful fungi, which process comprises treating the fungi or the plants, the soil or seeds to be protected against fungal attack, with an effective amount of at least one compound of the formula I, or an N-oxide, or an agriculturally acceptable salt thereof; to a process for preparing compounds of the formula I; to intermediate compounds of the formula II.a for the preparation of compounds of the formula I; to agrochemical compositions comprising at least one compound of the formula I; and to agrochemical compositions further comprising seeds.