Phenylacetic acid derivatives of the formula I ##STR1## where the substituents and the index have the following meanings: X is oxygen or sulfur;R is hydrogen or alkyl;R.sup.1 is hydrogen or alkyl;R.sup.2 is cyano, nitro, trifluoromethyl, halogen, alkyl or alkoxy;m is 0, 1 or 2, it being possible for the R.sup.2 radicals to be different if m is 2;R.sup.3 is hydrogen, cyano, nitro, hydroxyl, amino, halogen, alkyl, haloalkyl, alkoxy, haloalkoxy, alkylthio, alkylamino or dialkylamino;R.sub.4 is hydrogen, cyano, nitro, hydroxyl, amino, halogen,unsubstituted or substituted alkyl, alkoxy, alkylthio, alkylamino, dialkylamino, alkenyl, alkenyloxy, alkenylthio, alkenylamino, N-alkenyl-N-alkylamino, alkynyl, alkynyloxy, alkynylthio, alkynylamino, N-alkynyl-N-alkylamino;unsubstituted or substituted cycloalkyl, cycloalkoxy, cycloalkylthio, cycloalkylamino, N-cycloalkyl-N-alkylamino, cycloalkenyl, cycloalkenyloxy, cycloalkenylthio, cycloalkenylamino, N-cycloalkenyl-N-alkylamino, heterocyclyl, heterocyclyloxcy, heterocyclylthio, heterocyclylamino, N-heterocyclyl-N-alkylamino, aryl, aryloxy, arylthio, arylamino, N-aryl-N-alkylamino, hetaryl, hetaryloxy, hetarylthio, hetarylamino, N-hetaryl-N-alkylamino;R.sup.5 is hydrogen,unsubstituted or substituted alkyl, cycloalkyl, alkenyl, alkynyl, alkylcarbonyl, alkenylcarbonyl, alkynylcarbonyl or alkylsulfonyl;unsubstituted or substituted aryl, arylcarbonyl, arylsulfonyl, hetaryl, hetarylcarbonyl or hetarylsulfonyl,and their salts, processes and intermediates for their preparation, and their use are described.