The present invention relates to novel oxadiazoles of the formula I or an N-oxide and/or their agriculturally useful salts and to their use for controlling phytopathogenic fungi, or to a method for combating phytopathogenic harmful fungi, which process comprises treating the fungi or the materials, 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; and to mixtures comprising at least one such compound and at least one further pesticidally active substance selected from the group consisting of herbicides, safeners, fungicides, insecticides, and plant growth regulators; and to agrochemical compositions comprising at least one such compound and to agrochemical compositions further comprising seeds.