A synergistic microbicidal composition having two components. The first component is a nonionic surfactant with structure: R2O(CH2CH(CH3)O)3(CH2CH2O)5H, where R2 is a mixture of C8-C14 linear alkyl groups. The second component is 2-bromo-2-nitropropane-1,3-diol. The weight ratio of the nonionic surfactant to 2-bromo-2-nitropropane-1,3-diol is from 1:0.08 to 1:0.12 or 1:0.2 to 1:1.8286.