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Isolates of the endomycorrhizal fungus Hymenoscyphus ericae and the ectomycorrhizal fungus Hebeloma crustuliniforme from soils uncontaminated with AsO34, were compared with regard to their sensitivity to AsO34 in solution culture. When grown in liquid media amended with a range of AsO34 concentrations, H. ericae demonstrated reduced sensitivity to AsO34 compared to H. crustuliniforme. The concentrations causing 50% inhibition of growth (EC50) were 1.33 mol/m3 and 0.33 mol/m3, respectively, for H. ericae and H. crustuliniforme. The compound AsO34 is a PO34 analogue for the plasmalemma PO34 transporter. The presence of PO34in the media at high concentrations ameliorated the toxic effects ofAsO34 in both the ericoid and ectomycorrhizal fungi. This could be due to both suppression of the PO34 transporter under high phosporus status and competition of PO34 with AsO34 for the transport protein. The kinetics of AsO34 influx in H. ericae and H. crustuliniforme were also investigated. Hymenoscyphus ericae demonstrated a high Km value, 0.071 mol/m3, consistent with values obtained for AsO3-4tolerant plants. We suggest that the high Km value maybe a mechanism used by H. ericae to express reduced AsO34 sensitivity. The ecological significance of this reduced sensitivity is also discussed. [ABSTRACT FROM AUTHOR]