Disclosed is to a composition including difethialone, containing a configurational stereoisomer of difethialone, referred to as a hetero-stereoisomer, of formula 3-(4′-bromobiphenyl-4-yl)-1-(4-hydroxythiocoumarin-3-yl)-1,2,3,4-tetrahydronaphthalene, in which the carbon atoms 1 and 3 of the 1,2,3,4-tetrahydronaphthalene group have different absolute configurations, the composition including an amount of a dextrorotatory enantiomer of the hetero-stereoisomer such that the ratio of this amount to the amount of difethialone in the composition is less than 10%； the amount of the dextrorotatory enantiomer of the hetero-stereoisomer of difethialone in the composition being different from the amount of levorotatory enantiomer of the hetero-stereoisomer of difethialone in the composition.
