A synergistic herbicidal mixture comprising A) at least one 3-heterocyclyl-substituted benzoyl derivative of the formula I in which the variables have the following meanings: R1, R3 are halogen, alkyl, haloalkyl, alkoxy, haloalkoxy, alkylthio, alkylsulfinyl or alkylsulfonyl; R2 is a optionally substituted heterocyclic radical selected from the group: isoxazol-3-yl, isoxazol-4-yl, isoxazol-5-yl, 4,5-dihydroisoxazol-3-yl, 4,5-dihydroisoxazol-4-yl and 4,5-dihydroisoxazol-5-yl; R4 is hydrogen, halogen or alkyl; R5 is alkyl; R6 is hydrogen or alkyl; or one of its environmentally compatible salts; and B) at least the compound of formula IIa or one of its environmentally compatible salts; or the compound of formula IIb or one of its environmentally compatible salts; and, if desired, C) at least one further herbicidal compound; in a synergistically effective amount. Compositions comprising these mixtures, processes for the preparation of these compositions, and their use for controlling undesired plants.