Incidence of Armillaria root disease and the population structure of associated Armillaria spp. was studied in California mixed-hardwood forests. Four 3,000-m2 plots, consisting of Quercus kelloggii, Q. agrifolia, Lithocarpus densiflorus, Arbutus menziesii, Umbellularia californica, and Pseudotsuga menziesii, were intensively sampled. Root collars were examined for mycelial fans, decayed wood, and rhizomorphs. From 404 trees (333 living, 71 dead), Armillaria mellea and A. gallica were recovered and identified from 148 isolates. The most abundant tree species in each plot was the tree species most frequently infected by A. mellea or A. gallica. The majority of A. mellea isolates were recovered from mycelial fans on living trees. Recent mortality due to A. mellea was found only in one plot. A. gallica was mainly identified from epiphytic rhizomorphs. Only one to three somatic incompatibility groups (SIGs) for each species were found in each plot. Estimated sizes of SIGs varied from the extent of a single root system to the entire width of the plot. Based on our results, both A. mellea and A. gallica are common in mixedhardwood forests, yet A. mellea appears to be a more aggressive pathogen. A. gallica is capable of attacking live hosts, but occurs on living roots more frequently as epiphytic rhizomorphs.