This invention relates to synergistic combinations of selected microbicides in combination with a second microbicide or formulation ingredient or raw material. The combinations have greater efficacy than would be expected from combinations of the individual components. The combinations include mixtures of (a) 5-Chloro-2-methyl-4-isothiazolin-3-one+2-Methyl-4-isothiazolin-3-one, (b) Methyl-4-isothiazolin-3-one, or (c) 1,2-Benzisothiazolin-3-one with one or more of a variety of other compounds.