Methyl-substituted tetrahydro-2-pyrimidinone derivatives of the general formula ##STR1## in which each Ar, independently of each other, represents an aryl group which is optionally mono- or poly-substituted by substituent(s) selected from halogen, C.sub.1 to C.sub.6 alkyl, C.sub.1 to C.sub.6 alkoxy, C.sub.1 to C.sub.6 alkylthio, nitro, phenoxy and trifluoromethyl, andR.sup.1, R.sup.2 and R.sup.3 independently represent a hydrogen atom or a methyl group,provided that at least one of R.sup.1, R.sup.2 and R.sup.3 represents a methyl group,are new and find use as herbicides, in particular as selective herbicides which may be used on weeds in crops such as cotton and rice. The N,N'-diaryl-N-haloalkyl-ureas which are starting materials for the production of compounds of formula (I) are also new.