Substituted pyridines and pyrimidines, processes for their preparation and their use as pesticides and fungicides. The invention relates to compounds of the formula ##STR1## in which A is N or CH, R is H, halogen, alkyl or cycloalkyl, R.sup.2 and R.sup.3 are H, halogen or an aliphatic radical or together form a ring, X is O, NH Or S(O).sub.q where q=0, 1 or 2, R.sup.4.sub.q is 0-4 radicals selected from the series consisting of halogen, optionally substituted alkyl or alkoxy, cycloalkyl and optionally substituted phenyl, n=0-2, m=1-3 and Y is optionally substituted methylene or imino. The invention furthermore relates to a process for their preparation and for their use as pesticides, in particular as insecticides, acaricides and fungicides. The compounds are furthermore suitable for controlling nematodes, helminths and molluscs, and for controlling endoparasites and ectoparasites in the field of veterinary medicine.