A synergistic antimicrobial composition containing flumetsulam and an isothiazolone biocide chosen from among 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one, 2-n-octyl-4-isothiazolin-3-one and N-n-butyl-1,2-benzisothiazolin-3-one. A synergistic antimicrobial composition containing diclosulam and an isothiazolone biocide chosen from 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one and 2-n-octyl-4-isothiazolin-3-one.