A number of tropical soils growing toxic plant species which contain organically combined fluorine have been studied with a view to establishing the relationship between plant and soil in terms of fluorine. The analyses of the soils for total fluorine and water-soluble fluoride showed that there was no correlation with the levels of total and organically-combined fluorine in the leaves of the plant species. Some of the most toxic plants grew in soils having low levels of mineral fluorine. Examination by electrodialysis, gas-liquid chromatography and infra-red spectroscopy, supported by chemical analysis, has revealed the presence in significant amounts of what appears to be fluoroacetate in several of the soils supporting species known to contain toxic fluorinated fatty acids. The origin of the naturally-occurring fluoroacetate in these soils is discussed.