Microbicidally active novel 1-heteroaryl-4-aryl-pyrazolin-5-ones of the formula ##STR1## in which R represents hydrogen or alkyl,R.sup.1 represents halogen, hydroxyl, optionally substituted alkyl, optionally substituted alkoxy, optionally substituted phenoxy, a radical --S(O).sub.p -- alkyl which is optionally substituted in the alkyl part, nitro, optionally substituted amino or a fused-on carbocyclic or heterocyclic radical,p represents an integer 0, 1 or 2,R.sup.2 represents halogen or optionally substituted alkyl, or represents alkoxy, alkylmercapto, nitro, cyano, carboxamide or a fused-on carbocyclic radical,m represents an integer from 0 to 5,n represents an integer from 0 to 4, it being possible for the substituents to be identical or different when m and/or n represent a number greater than 1, andX, Y and Z are identical or different and represent a nitrogen atom or the radical .dbd.CH or ##STR2## wherein R.sup.2 has the meaning given above, with the proviso that at least one of the radicals X, Y and Z denotes a nitrogen atom.