A method for treating water in a closed cooling system, having a pH of at least 7 and a temperature of at least 25.degree. C., by adding a composition comprising 2-methyl-4-isothiazolin-3-one; wherein the composition is substantially free of non-aqueous solvents, volatile organic compounds and halogenated biocides; and wherein 2-methyl-4-isothiazolin-3-one is present in an amount sufficient to produce in the water a 2-methyl-4-isothiazolin-3-one concentration from 15 ppm to 500 ppm.