The application describes synergistic microbicidal compositions comprising: 2,2-dibromomalonamide and an oxidizing biocide selected from the group consisting of: monochloramine, bromochlorodimethylhydantoin, hypobromite ion or hypobromous acid, hydrogen peroxide, dichloroisocyanurate, trichloroisocyanurate and chlorine dioxide and a method for controlling microorganism growth in an aqueous or water-containing system.