A synergistic antimicrobial composition having two components. The first component is a hydroxymethyl-substituted phosphorus compound. The second component is one of the following biocides: hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine, 2,6-dimethyl-1,3-dioxan-4-yl acetate or ortho-phenylphenol or its alkali metal or ammonium salts.