Genetic individuals, or genets, of Armillaria and other root-infecting basidiomycetes are usually found in discrete patches that often include the root systems of several adjacent trees. Each diploid individual is thought to arise in an unique mating event and then grow vegetatively in an expanding territory over a long period of time. Our objective in this study was to describe the population from which such genetic individuals are drawn. In a sample including 274 collections representing 121 genetic individuals of A. gallica (synonym A. bulbosa) from two sites in each of four regions of eastern North America, genotype frequencies at seven nuclear loci were not significantly different from Hardy-Weinberg expectations. Furthermore, allele frequencies at the seven loci were not significantly different between regions. Additional allelic data from four non-contiguous regions of mitochondrial DNA showed little or no population subdivision over the four regions. Analysis of the distribution of multilocus mtDNA haplotypes revealed some clonal transmission of mtDNAs between genets and nonrandom mating within sites. Despite the sharing of mtDNA types by some individuals, the overall sample contained a high level of genotypic diversity. The apparent linkage equilibrium between some pairs of loci and the high level of phylogenetic inconsistency among all four loci suggest the occurrence heteroplasmy and recombination among mtDNAs of A. gallica in nature. In laboratory matings of two haploid strains with different mtDNA types, a low frequency of recombination in mtDNA was detected.