The intergenic spacer (IGS) region, which is located between the 3' end of 26S ribosomal DNA (rDNA) and the 5' end of 5S rDNA, of six Armillaria species from Hokkaido was investigated using polymerase chain reaction-restriction fragment length polymorphisms (PCR-RFLP). Restriction with only Alu I could distinguish A. mellea subsp, nipponica from the other species. With Alu I and Dde I, A. ostoyae and A. gallica could be distinguished from the other species. Digestion with Alu I resulted in two patterns (types A and B) of A. singula and three patterns (types A, B, and C) of A.[ezoensis. One pattern (type B) of the former species and two patterns (types B and C) of the latter species were each different from those of the other species. Armillaria sinapina gave only one Alu I digestion pattern, which was identical to that of A. jezoensis (type A) and A. singula (type A). However, by digestion with Dde I, A. singula (type A) could be distinguished from A. jezoensis (type A) and A. sinapina.