Use of dithiine derivatives (I) for controlling undesirable microorganisms, preferably phytopathogenic fungi, in crop protection and material protection, and in the area of household and hygiene, is claimed. Use of dithiine derivatives of formula (I) for controlling undesirable microorganisms, preferably phytopathogenic fungi, in crop protection and material protection, and in the area of household and hygiene, is claimed. Either R1-R4 : (hetero)aryl or (hetero)aryl-(1-4C-alkyl) (both optionally substituted by one or more halo, nitro, cyano, 1-4C-(halo)alkyl or 1-4C-alkoxy), H, halo, nitro, 1-8C-(halo)alkyl, 1-8C-(halo)alkoxy, 1-8C-(halo)alkylthio, -COR5, -CH 2COR5 or N-phenylmorpholin-4-carboximidoyl； and R5 : hydroxy, 1-4C-alkyl or 1-4C-alkoxy； and m, n : 0-2； or R1+C+R2 : 5- or 6-membered heterocycle having one or two heteroatoms comprising N, S or O, where the heterocycle contains one or two C(=O)- or (C=S)- groups, and is optionally substituted by 1-4C-alkyl, -COR6 or di(1-4C-alkyl)amino, and when the heterocycle is pyrazine then the pyrazine is substituted by one or two 1-4C-alkyl, -COR6 or di(1-4C-alkyl)amino, and the heterocycle is not 1,3-dithio-2-(thi)one-ring； and either R3, R4 : H or R1 and R2； or R3+C+R4 : an aryl ring； and R6 : hydroxy, 1-4C-alkyl or 1-4C-alkoxy, where both R1 and R2, and R3 and R4 are not optionally substituted 1H-pyrrol-2,5-dione-ring； and m, n : 0； or R1, R2 : -C(=O)-X1-C(=O)-； and R3, R4 : -C(=O)-X2-C(=O)-； and X1 : S or NR7； and X2 : S or NR7a, or phenyl-1,2-diyl； and R7 : 1-8C-(halo)alkylthio, 1-4C-alkylamino, di-(1-4C-alkyl)amino, phenylsulfonylamino or aryl or aryl-(1-4C-alkyl) (both substituted by three halo, 1-4C-(halo)alkyl, 1-4C-(halo)alkoxy or 1-8C-alkylthio)； and R7a : R7 or H； and m, n : 0； or R1R2 : -C(=O)-CR8=CR9-C(=O)-； and either R3, R4 : cyano； or R3R4 : -C(=O)-CR8=CR9-C(=O)-； and R8, R9 : H, halo, 1-8C-alkyl, 1-8C-(halo)alkoxy or 1-8C-(halo)alkylthio, where at least one R8 or R9 is not H, when both R1 and R2, and R
