The tissues of numerous species of toxic plants from Africa, Australia and South America have been analysed for organic and inorganic fluorine. The presence of organically combined fluorine was established in most of the species but was very variable in concentration. The stems and roots of the African Dichapetalaceae generally contained much more fluorine than did the leaves and the reversed situation was found in the same tissues of the Australian Gastrolobium and Oxylobium species and in the Brazilian Palicourea marcgravii. The seeds of most of the specimens analysed contained large amounts of organic fluorine but was absent in those of Acacia georginae. None of the plant species examined grew in soils which were high in fluorominerals; on the contrary some of the plants grew in soils containing exceptionally low levels of fluorine. A waxy exudate on the outside of the toxic leaves of A. georginae contained significant amounts of organic fluorine and is believed to be part of an excretory mechanism. Only a small proportion of the exudate was identified as fluoroacetate and there appear to be other fluorinated organic compounds present. A crystal formation which contains fluorine was observed in the bark of the roots and stems of Dichapetalaceae and is thought to be also associated with a detoxication process. The crystals appear to be a complex mixture containing calcium oxalate, chloride, fluorine, magnesium and silica. The analytical findings suggest that in the plants in which carbon-fluorine compounds have been established, the site of the synthesis of organic fluorine may vary with the species and that metabolism to inorganic fluoride may take place continuously and simultaneously with the organic bio-synthesis.