The biological potential of some South Island soils for defluorination of sodium monofluoroacetate (NaFa) was examined. In these soils were found a diversity of bacteria: Pseudomonas sp., Bacillus subtilis, Aeromonas sp., Actinobacillus sp., Citrobacter sp., Staphylococcus aureus and Lactobacillus Spa Also found were the fungi: Acremonium strictum, Fusarium solani, and F. oxysporum which showed the ability to cleave the carbon-fluoride bond of fluoroacetate. NaFa-tolerant organisms were also isolated from these soils and include Chlorella sp., and the fungi: Penicillium sp., Aspergillus sp., and Mucor sp. Of these organisms, the fungi were selected for more detailed study of the influence of environmental factors on their growth and defluorinating activity. Factors which favoured growth also favoured the defluorination of NaFa. Growth of F. solani and its defluorination of NaFa increased with increasing temperature from 10°c to 30°c. While growth of F. solani showed an optimal pH plateau from 5.8 - 7.2, defluorination of NaFa was more sensitive to external pH with a sharp optimum at 5.8. Studies on the effect of nitrogen sources (NO-3 , NH+4, and urea) showed that both the growth and defluorination of NaFa were maximal in NH+4-N medium up to 448 mg N 1-¹ whereas such effect on A. strictum was not evident. Further increase in NH+4-N concentration was inhibitory to both growth and NaFa defluorination. The availability of additional carbon sources (acetate, glucose) enhanced growth and NaFa defluorination. The degree of enhancement of defluorinating activity decreased with increasing concentration of the supplementary carbon source in NaFa-medium for F. Solani, whereas the response of Penicillium sp., in terms of growth and NaFa de fluorination increased with increasing glucose concentration. The ability of fungi to degrade NaFa was induced. From assays of the culture filtrate, and various cell fractions, it was shown that de fluorination of NaFa took place intracellularly. The regulation of growth and de fluorination of NaFa by environmental factors is discussed. Studies of NaFa de fluorination were pursued at cellular and enzymatic levels. The defluorinating enzyme, haloacetate halidohydrolase or fluorohydrolase, extracted from F. solani and Pseudomonas Spa were similar in their molecular weight, estimated to be between 45,000 - 68,000. Both enzymes catalysed the breakdown of NaFa to F- and glycolate on a mole to mole basis. The fluorohydrolase was highly specific to monohalogenated acetates, showing a higher specific for fluoroacetate and fluoroacetamide. Fluorohydrolase activity was inhibited by thiol-alkylating agents suggesting the involvement of -SH groups at the active site of the enzyme. The mechanism of enzymatic defluorination of NaFa was discussed. The Q10 values and the response to pH of NaFa defluorination by cells and enzyme were different and their difference was discussed in terms of ionisation of fluoroacetate and their rate of diffusion into the cells. Phytotoxicity studies with Chlorella sp., and three species of duckweeds: Spirodela oligorrhisa, S. polyrrhina and Lemna minor showed that the alga was highly tolerant while the duckweeds were extremely sensitive to the presence of NaFa. Whilst growth of Chlorella Spa was unaffected in 20mM NaFa medium, growth of the duckweeds was completely suppressed in 0.5-1.0 mM NaFa medium. The tolerance or susceptibility of plants to NaFa is discussed.