Ichthyotoxic (piscicidal) substances isolated from natural sources often exhibit other biological and pharmaceutical properties such as insecticidal, (anti) tumor-promoting, antibacterial and (anti)inflammatory activities. Ichthyotoxic assay for plant extractives would thus be useful as a primary screening method in the search for bioactive natural products. In a study to verify this supposition, ichthyotoxicity-guided fractionation using killie-fish (Oryzias latipes; medaka) for several plant species has been investigated and led to the isolation and characterization of bioactive substances including new compounds (terpenoids, phloroglucinols and phenolic derivatives). Iridal-type triterpenoids isolated as toxic principles to killie-fish from iridaceous plants, contained anti-ulcer components tested in experimental animals, and/or tumor promoters. Some of the ichthyotoxic phloroglucinol derivatives and a sesquiterpene from Dryopteris fragrans and Inula helenium, respectively, showed in vitro and in vivo antitumor-promoting activity. These toxic compounds except for the new compounds were also reported as potent antibacterial, anthelminthic and anti-tumor agents.