Found on wetlands, in ponds and rivers, representatives of the family Ranunculaceae inhibit the growth of several species of fungi and fungus-like organisms. Of the eight species of the Ranunculaceae examined, Ranunculus sceleratus was found to most markedly inhibit the growth of the mycota, whereas Batrachium fluitans showed the least inhibitory effect. Probably the inhibitory factors are protoanemonin and anemonin-type lactones, which when excreted to the aquatic environment inhibit the growth of several mycota species.