An O-aryl-N'-(triazin-2-yl)-isourea of the formula ##STR1## in which R.sup.1 represents the radical ##STR2## wherein R.sup.5 represents trifluoromethoxy, difluoromethoxy, phenyl, phenoxy, trifluoromethylthio, difluoromethylthio, di-(C.sub.1 -C.sub.4 -alkyl)-aminocarbonyl, C.sub.1 -C.sub.4 -alkylsulphonyl, C.sub.1 -C.sub.4 - alkoxyaminosulphonyl, N-(C.sub.1 -C.sub.4 -alkoxy)-N-(C.sub.1 -C.sub.4 -alkyl)-aminosulphonyl, C.sub.1 -C.sub.4 -alkylaminosulphonyl or di-(C.sub.1 -C.sub.4-alkyl)-aminosulphonyl,or whereinR.sup.1 represents the radical ##STR3## wherein R.sup.6 represents hydrogen or C.sub.1 -C.sub.4 -alkyl andR.sup.7 and R.sup.8 are identical or different and represent hydrogen, fluorine, chlorine, bromine, nitro, cyano, C.sub.1 -C.sub.4 -alkyl (which is optionally substituted by fluorine and /or chlorine), C.sub.1 -C.sub.4 -alkoxy (which is optionally substituted by fluorine and/or chlorine), carboxyl, C.sub.1 -C.sub.4 -alkoxycarbonyl, C.sub.1 -C.sub.4 -alkylsulphonyl or di-(C.sub.1 -C.sub.4 -alkyl)-aminosulphonyl;R.sup.2 represents optionally substituted aryl,R.sup.3 represents hydrogen, halogen, hydroxyl, C.sub.1 -C.sub.6 -alkylamino or di-(C.sub.1 -C.sub.6 -alkyl)-amino, or represents optionally substituted C.sub.1 -C.sub.6 -alkyl, C.sub.1 -C.sub.6 -alkoxy or C.sub.1 -C.sub.6 -alkylthio andR.sup.4 respresents hydrogen, halogen or hydroxyl, or represents optionally substituted C.sub.1 -C.sub.6 -alkyl, C.sub.1 -C.sub.6 -alkoxy or C.sub.1 -C.sub.6 -alkylthio, or represents C.sub.1 -C.sub.6 -alkylamino or di-(C.sub.1 -C.sub.6 -alkyl)-amino.