Abstract The mixed fatty acids obtained from the seeds of Dichapetalum toxicarium by hydrolysis have been fractionated by low.temperature separation from organic solvents followed by reversed.phase chromatography. After further purification by a defined calcium precipitation, an oil of m.p. 13.0° C ± 1.0 was separated. The main constituent was a long.chain fluoroacid containing eighteen C atoms, one double bond and one F atom, i.e. a fluoro.octadecenoic acid. In biochemical tests with kidney particles, As little as 1.25 /smg of the fluoro fatty acid led to citric acid accumulations. Some of the tests upon rats are recorded; it was found that at death there was usually more citric acid in the heart than in the kidney, which is the reverse of the picture with fluoroacetate. In terms of fluorine content, the long.chain fluoro.fatty acids are more toxic than fluoroacetate.