New heterocyclic-substituted 1,3,4,-thiadiazole derivatives of the formula ##STR1## in which R.sup.1 is alkyl, alkenyl, haloalkyl, optionally substituted aralkyl, optionally substituted aryl, alkylthio, alkylsulfonyl, alkylsulfoxyl, aralkylthio, aralkylsulfonyl or aralkylsulfoxyl,R.sup.2 is methyl or ethyl, andn is 1 or 2,are surprisingly effective as herbicides and are particularly useful as selective herbicides for combating grasses, especially in crops of cereals or cotton.