Many marine lives prevent the surface of their bodies with antifouling substances without causing serious environmental problems. Some terrestrial plants also have the same kinds of substances. Therefore, these substances may be expected to be utilized as new environmentally friendly antifouling agents, especially those having high anesthetic, repellent, settlement deterrent, settlement inhibitory properties, etc., without having biocidal properties. These natural products are classified into five kinds of compounds, i.e., terpenes, nitrogen-containing compounds, phenols, steroids and others. Many compounds such as sesquiterpenes, diterpenes, trigonelline, primidines, macrocarpals and higher fatty acids, showed the same level of antifouling properties as that of CuSO 4, and some compounds such as gramines, cycloviolacin O2, shogaols, alkylphenols, kaempferols and bufalin, showed the similar high antifouling properties as those of the organotin antifoulants. The mixtures of these compounds are expected to be much better antifoulants than the organotin compounds since some of them showed synergistic properties.