Mating experiments were conducted with 30 California isolates of Armillaria species to determine their intersterility grouping. Basidiomes were collected from a variety of coniferous and hardwood hosts in urban, agricultural and forested regions in northern and central California. Monosporous cultures were obtained and paired with haploid tester strains representing 10 North American biological species. A majority of the California isolates were compatible with Armillaria mellea sensu stricto (= North American biological species VI). The exceptions were four isolates from a low elevation, coastal forest. Three of these were compatible with North American biological species IX, a still unidentified Armillaria species, and one was compatible with A. gallica (= North American biological species VII). These findings show that Armillaria species diversity exists in California, but that A. mellea predominates on several hosts and in diverse habitats.