A synergistic microbicidal composition having two components. The first component is a nonionic surfactant with structure: R1O(CH2CH(CH3)O)5(CH2CH2O)9H, where R1 is a C8 alkyl group. The second component is 2-methyl-4-isothiazolin-3-one. The weight ratio of the nonionic surfactant to 2-methyl-4-isothiazolin-3-one is from 1:0.005 to 1:1.3714.