A finely divided biocidal substance is maintained very fine and stable in an aqueous suspension over a long period of storage or cyclical storage between low and high temperatures by adding to said aqueous suspension a particle growth inhibitor comprising:
(1) A water-soluble or water-dispersible polymer containing as its essential constituent at least one compound selected from the monomer group consisting of unsaturated carboxylic acids and their derivatives,
(2) A water-soluble or water-dispersible polymer containing styrene sulfonates as its essential constituent, or
(3) A formalin condensate or a salt thereof of sulfonation products of a polycyclic aromatic compound which may contain a hydrocarbon group as a substituent.