The 1-(3,5-bis-trifluoromethylphenyl)-2-thioxopyrrolidine-4-carboxylic acid derivatives of the formula I below are suitable for protecting crop plants against the phytotoxic action of herbicides and for regulating the plant growth.The 1-(3,5-bis-trifluoromethylphenyl)-2-thioxopyrrolidine-4-carboxylic acid derivatives are those of the formula I ##STR1## wherein A is --COOR.sub.1, --COSR.sub.1, --COO.sup..crclbar. M.sup..sym., --CONR.sub.2 R.sub.3 or --COCl; R.sub.1 is hydrogen, C.sub.1 -C.sub.4 alkyl, C.sub.2 -C.sub.6 alkenyl or C.sub.2 -C.sub.6 alkynyl; R.sub.2 and R.sub.3 independently of one another are hydrogen, C.sub.1 -C.sub.4 alkyl or C.sub.3 -C.sub.7 cycloalkyl; or R.sub.2 and R.sub.3 together with the nitrogen atom to which they are bonded are a saturated 3- to 7-membered heterocycle which can contain an additional hetero atom selected from the group comprising O, N and S and which is unsubstituted or up to trisubstituted by C.sub.1 14 C.sub.4 alkyl; and M.sup..sym. is the equivalent of an alkali metal cation or an alkaline earth metal cation or HN.sup..sym. (R.sub.2).sub.3, and their isomers in optically pure or enriched form.