ABSTRACTThis study used restriction fragment polymorphisms in nuclear and mitochondrial DNA to confirm what had been surmised from the studies of mating interactions among isolates ofArmillaria mellea(broad sense): that the“biological species”are distinct entities among which genetic divergence has occurred. We examined the DNAs of 16 isolates representing 8 biological species. Three different types of DNA were used as probes in Southern hybridizations with EcoRI-digested, whole-cell or nuclear DNAs: i) total mitochondrial DNA from a strain of biological species I, ii) ten bacterial plasmid clones carrying randomEcoRIfragments of nuclear DNA from a strain of biological species I, and iii) a plasmid clone carrying the entire rDNA repeat fromSehizophyllum commune.Most of the restriction fragments identified were variable in the sample and most of the variability was between, rather than within, biological species.