The present invention relates to a process for preparing 2-(aminomethylidene)-4,4-dihalo-3-oxobutyric esters of the formula (I), wherein R1, R2, R3 are C1-C6-alkyl, C1-C6-haloalkyl, C2-C6-alkenyl, C3-C10-cycloalkyl or benzyl, and/or R2 together with R3 and the nitrogen atom to which the two radicals are attached are a heterocyclic radical, in which a corresponding 3-aminoacrylic ester is reacted with a halogen-substituted acetyl fluoride in the presence of at least one alkali or alkaline earth metal fluoride; and the further conversion of halogen-substituted 2-(aminomethylidene)-3-oxobutyric esters of the formula (I) to halomethyl-substituted pyrazol-4-ylcarboxylic acids and their esters.