A composition for controlling harmful bio-organisms comprising (a) at least one imidazole compound represented by formula (I) (see formula I) wherein R represents a lower alkyl group or a lower alkoxy group; and n represents an integer of 1 to 5, as an active ingredient, and (b) at least one fungicide for Phycomycetes as an active ingredient, and a method for controlling harmful bio-organisms comprising applying the composition for controlling harmful bio-organisms onto harmful bio--organisms.