The present invention relates to co-crystals of cyprodinil or pyrimethanil and a co-crystal forming compound which has at least one organic acid functional group. In particular, the present invention relates to co-crystals of cyprodinil and benzoic acid, succinic acid, fumaric acid, maleic acid, oxalic acid, pyrazine carboxylic acid, glycolic acid, levulinic acid, (2-methylphenoxy)acetic acid, hexanedioic acid, 4-(methylamino) benzoic acid, trimethyl acetic acid, pyruvic acid or 4-hydroxy-4′-biphenyl carboxylic acid.