A method for treating water in a closed cooling system, having a pH of at least 7 and a temperature of at least 25°C, by adding a composition comprising 2-methyl-4-isothiazolin-3-one wherein the composition is substantially free of nonaqueous 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.
