The main aim of this project was to investigate the role of Fusarium solani in aqueous cutting fluid and increase understanding about the ecology of this organism in coolants. By adapting cell-line bioassay techniques to the direct testing of coolant emulsion, the role of Fusarium solani in cutting fluid toxicity was determined. The potential of fungal strains enhancing cutting fluid toxicity was investigated by fungal toxigenicity screening, to assess any potential for fungal species isolated to produce mycotoxins. 32 contaminated cutting fluid samples tested revealed 66 % fungal contamination and Fusarium solani was isolated from 81 % of these, highlighting the significance of this species in coolants. Using brine shrimp and cell-line bioassays, none of the fungi isolated were toxigenic, although Fusarium solani culture extracts were slightly toxic to cell-lines (min. cytotoxic dose 12.5-25 ug/ml). Using thin layer chromatography, it was revealed that the isolate Fusarium solani ISL-45 / IMI 360547 was not producing trichothecenes in culture, and that a maroon pigment normally produced at pH < 4 was responsible for slight cytotoxicity. It was revealed that pigment production was only stimulated in cultures less than pH 4 in the presence of excess phosphate. By assaying cutting fluid using cell-lines it was found that growth of Fusarium solani reduced cutting fluid toxicity twofold after 14 days growth and that fungal biodeterioration of vegetable oil and ethylene glycol was responsible for the toxicity decrease. Cutting fluid not supplemented with yeast extract or other nitrogenous material was unable to support growth of Fusarium solani demonstrating that there was an obligate requirement for an external assimilable nitrogen source in addition to nitrogenous cutting fluid constituents. Fusarium solani biomass yields were highest in cutting fluid experiments supplemented with iron / aluminium swarf and inert beads. Biomass yields were poor in cutting fluid containing no swarf or inert material suggesting that surfaces promote fungal adhesion and accumulation of mycelium. Although swarf metal was stimulatory to growth, cutting fluid treated with dissolved swarf materials was inhibitory to growth. II In metal ion tests; Ca2+, Mg2+ and Mn2+ had little effect on fungal physiology whereas A13+, Cu2+, Fe3+, Ni2+ and Ti3+ were very toxic (min. inhibitory dose M.I.D. < 2mM) and C~+, Co2+, Fe2+, Pb2+ and Zn2+ were moderately toxic (M.I.D. 2-10 mM). 0.5 mM Pb2+ and Zn2+ were associated with increased production of maroon pigment, by Fusarium solani, in media containing glucose. Water hardness was found to have little or no effect on fungal physiology although coolant emulsion stability was reduced with Ca2+ concentrations exceeding 5 mM.