A synergistic microbicidal composition. The composition comprises a silver-containing copolymer comprising polymerized units of a monomer X and a monomer Y; wherein monomer X is an ethylenically unsaturated compound having a substituent group selected from an unsaturated or aromatic heterocyclic group having at least one hetero atom selected from N, O and S; wherein monomer Y is selected from carboxylic acids, carboxylic acid salts, carboxylic acid esters, organosulfuric acids, organosulfuric acid salts, sulfonic acids, sulfonic acid salts, phosphonic acids, phosphonic acid salts, vinyl esters, (meth)acrylamides, C8-C20 aromatic monomers containing at least one exocyclic ethylenic unsaturation and combinations thereof; and at least one organic biocide.