A β-1,4-endoxylanase from the ericoid mycorrhizal fungus H. ericae has been purified to electrophoretic homogeneity using isoelectric focusing, ion exchange and gel permeation chromatography. The enzyme has an isoelectric point of 4·85–5·20 and a molecular weight of 58·4 kDa. The apparent S0·5 of the enzyme for soluble birchwood glucuronoxylan is 3·75 mg ml−1 and the Vmax 468·0 nkatal mg−1 protein. The pH optimum for activity is 4·5 and that for stability is 3·5–4·0; these values are discussed in the context of the pH of the mor humus. The role of wall-degrading activities in the establishment of the ericoid mycorrhizal symbiosis is considered.