Some Armillaria species cause major disease problems in both forestry and gardens. In order to determine routinely and rapidly the identity of an isolate, a PCR-based method was investigated. A fragment, approx. 900 bp, of the IGS region of the ribosomal DNA was amplified from 96 different isolates of six species of Armillaria collected from the U.K. and mainland Europe. Armillaria mellea, A. gallica and A. tabescens produced unique digestion patterns when the PCR fragment was restricted with Alu I. A. ostoyae could be distinguished from A. borealis and A. cepistipes when the PCR fragment was restricted with Bsm I. Previously unreported RFLP patterns were determined for A. mellea and A. gallica. The existence of A. gallica isolates which are putative heterozygotes for the rDNA cluster is also reported. Based on differences in banding patterns. A. mellea, A. gallica, A. cepistipes and A. borealis were subdivided further into groups which correlated with differences in cultural morphology.