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 - 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 -- 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 .