Thiocarboxylic acid derivatives useful as herbicides or plant growth regulators are of the general formula I ##STR1## where R.sup.1 is unsubstituted or substituted alkyl, unsubstituted or substituted cycloalkyl or cycloalkenyl, unsubstituted or substituted alkenyl or alkynyl or unsubstituted or substituted phenyl;R.sup.2 is halogen, alkyl, haloalkyl, alkoxy, haloalkoxy or alkylthio;X is nitrogen or CR.sup.6, where R.sup.6 is hydrogen or, together with R.sup.3, forms a C.sub.3 - or C.sub.4 -alkylene chain or C.sub.3 - or C.sub.4 -alkenylene chain in which a methylene group is replaced with oxygen;R.sup.3 is halogen, alkyl, haloalkyl, alkoxy, haloalkoxy or alkylthio or R.sup.3 is bonded to R.sup.6 as stated above to give a 5-membered or 6-membered ring;R.sup.4 is unsubstituted or substituted alkyl, unsubstituted or substituted alkenyl or alkynyl, an unsubstituted or substituted 5-membered or 6-membered heteroaromatic structure containing from one to three nitrogen atoms and/or one sulfur or oxygen atom, or unsubstituted or substituted phenyl or naphthyl;R.sup.4 and R.sup.5 together with the adjacent carbon atom form a 3-membered to 6-membered ring which may contain an oxygen or sulfur atom and may be substituted;R.sup.5 is hydrogen, alkyl, haloalkyl, alkoxy, haloalkoxy, or alkylthio;and Y and Z are each oxygen or sulfur.