Novel usage

Usage of a compound of formula (I)

• 6
• or of one of its physiologically tolerable acid addition salts, wherein X is halogen;
• X1 is hydrogen or halogen;
• X2 is hydrogen or halogen;
• Y is partially or wholly halogenated C1-C6-alkyl;
• or partially or wholly halogenated C1-C6-alkyl interrupted by an oxygen atom;
• or partially or wholly halogenated C2-C6-alkenyl;
• or if —O—Y is in position 3, represents the group
• 7
• Y1 is hydrogen or halogen;
• Y2 is hydrogen or halogen;
• Y3 is hydrogen or halogen;
• Z1 is hydrogen or C1-C3-alkyl;
• Z2 is hydrogen or C1-C3-alkyl;
• and U is hydrogen or halogen;
• with the exception of 1-[3,5-dichloro-4-(1,1,2,2-tetrafluoroethoxy)phenyl]-(2,6-difluorobenzoyl)-urea, as an active substance in a method of controlling a sea lice infestation on fish.