3-(Het)arylcarboxylic acid derivatives of the formula I ##STR1## where R is formyl, CO.sub.2 H or a radical hydrolyzable to COOH and the other substituents have the following meanings: R.sup.2 and R.sup.3 are each halogen, alkyl, haloalkyl, alkoxy, haloalkoxy or alkylthio; X is nitrogen or CR.sup.14, where R.sup.14 is hydrogen or, together with R.sup.3, forms an alkylene or alkenylene chain, in each of which a methylene group is replaced by oxygen; R.sup.4 is phenyl or naphthyl, each of which is unsubstituted or substituted or an unsubstituted or substituted five-membered or six-membered heteroaromatic structure containing one to three nitrogen atoms or one sulfur or oxygen atom; R.sup.5 is hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, haloalkyl, alkoxyalkyl, alkylthioalkyl or phenyl; R.sup.6 is C.sub.1 -C.sub.8 -alkyl, C.sub.3 -C.sub.6 -alkenyl, C.sub.3 -C.sub.6 -alkynyl or C.sub.3 -C.sub.4 -cyclo-alkyl, each of which may be mono- or polysubstituted; Y is sulfur, oxygen or a single bond; and Z is sulfur or oxygen; with the proviso that R.sup.6 is not unsubstituted C.sub.1 -C.sub.4 -alkyl when R.sup.4 is unsubstituted phenyl, Z is oxygen and simultaneously R.sup.5 is methyl or hydrogen.