Four forests each of 4–8 ha were investigated in Central France: (1) a pure beech stand harbouring only the saprophytic species Armillaria gallica and A. cepistipes; (2) a pure beech stand with the same species plus A. ostoyae; (3) an ancient beech stand to which conifers had been introduced 60 yr ago, and in which A. ostoyae was becoming the dominant species with A. gallica and A. cepistipes still present; (4) a young pine stand on an old conifer site, where A. ostoyae was exclusive and pathogenic. A total of 764 isolates was obtained from rhizomorphs, fruiting bodies and mycelia from stumps. Mapping of genets of the different Armillaria species was carried out on the four sites by testing for somatic incompatibility. This allowed comparison of the number, area and distribution of the genets of the different species. There was complete overlap of the genets of A. ostoyae with those of A. cepistipes or A. gallica, suggesting different colonization strategies. By contrast, the genets of A. gallica and A. cepistipes overlapped only rarely, suggesting similar colonization strategies. Two different genets of the same species intermingled only in a limited area along their border. In the conifer plantation, A. ostoyae tended to colonize the stumps whereas the other two species prevailed as rhizomorphs in the litter, but the competition was evolving in favour of A. ostoyae. At the fourth site, where the stumps had been uprooted and the pine seedlings had been left to grow, a large number of small genets of A. ostoyae was observed, suggesting recent infection by basidiospores.