The effects of the metals, iron and aluminium, on acid phosphatase (P-NPPase) activity of the ericoid endophyte, Hymenoseyphus ericae (Read) Korf and Kernan were determined. The endophyte was grown in pure culture and then treated with Fe2+ and Al2−over a range of concentrations (0–1000mgl−1), and at two concentrations of inorganic phosphorus (20μ and 200 μM p). Activities of p-NPPase were determined. Enzyme activity was greatly inhibited at the highest concentration of external P even in the absence of the metal ions. The presence of the metals in the low P treatment had effects determined by their concentration. At the highest concentration used, they precipitated products of acid phosphatase activity (p-NP), and so invalidated the use of the assay at these metal levels. However, the assay could reliably be used over the concentration range 0–200 mg 1−1 Al2+ and 1–100 mg 1 −1 Fe2+, the upper concentrations in both cases being considerably above those normally found in soil solutions. Over these ranges the presence of Al3+ only slightly inhibited p-NPPase activity, whereas there was a small increase of activity in the case of Fe2+. The results are discussed in relation to the role of ericoid mycorrhizal infection in the phosphorus nutrition of ericaceous plants in natural ecosystems.