Abstract: Use of 4-alkyl-substituted diaminopyrimidines of the formula (I) as fungicide in which R1 to R13 and X1 and X2 have the meanings given in the description, and agrochemically active salts thereof, and also methods and compositions for controlling phytopathogenic harmful fungi in and/or on plants or in and/or on seed of plants, processes for preparing such compositions and treated seed and also their use for controlling phytopathogenic harmful fungi in agriculture, horticulture and forestry, in the protection of materials and in the domestic and hygiene field. The present invention furthermore relates to a process for preparing diaminopyrimidines of the formulae (Ia), (Ib) and (Ic).