Highly concentrated, water-based dispersions of certain lipophilic and/or hydrophobic antimicrobially active materials are stabilized to a surprising degree by a surfactant combination including a nonionic acrylic graft copolymer surfactant and a alkoxylated polyarylphenol phosphate ester surfactant. The active materials may be present in dispersion concentrates of the present invention singly or in useful combinations. The active materials are selected from the group of fungicides and bactericides consisting of 1,2-benzisothiazol-3(2H)-one; 2-octyl-2H-isothiazol-3-one; 5-chloro-2-methyl-2H-isothiazol-3-one; 2-methyl-2H-isothiazol-3-one; pyrithione zinc; 3-iodo-2-propynyl butylcarbamate; 2-methylthio-4-ethylamino-6-tert-butylamino-s-triazine; and 3-(4-isopropylphenyl)-1,1-dimethylurea, and mixtures thereof. The dispersion concentrates may be efficiently shipped and stored, and subsequently diluted with water to produce less concentrated dispersions when desired. Even at relatively high concentrations, the dispersion compositions of the present invention can be stored for months or years without significant loss of stability.