Abstract: The Armillariella mellea complex in Finland is divided on the basis of mating experiments into three completely intersterile biological species with different morphological and ecological characteristics. Their nomenclature is still unclear. They are intersterile with A. mellea (Vahl ex Fr.) Karst. sensu stricto and A. bulbosa (Barla) Romagn., collected in France. All these five species exhibit a heterothallic bifactorial mating system, and in four species a similar mechanism of somatic diploidization in compatible mating has been observed. The diploid pure cultures belonging to these species can be identified in mating tests with the aid of the Buller phenomenon. Two species are common in Finland. They are predominantly saprophytes and occur to some extent in the butt rot of Picea abies. The third species is a pathogen of Pinus sylvestris saplings and has been found only in the southern part of the country. Some information about the occurrence of these species outside Finland is presented. The naturally occurring clones of Armillariella species have been identified by using incompatibility factors as genetic markers and on the basis of demarcation line formation in confrontations between diploid pure cultures. The results obtained with these two methods are in good agreement. The usual diameter of Armillariella clones in Finland is 10-50 m. The largest clones found had a diameter of 120-150 m.