The spatial distribution of genets in the ectomycorrhizal fungus Suillus bovinus (L.: Fr.) Roussel were studied in somatic incompatibility pairings of isolates from five Scots pine (Pinus sylvestris L.) stands differing in forest history and age, With increasing forest age, the size of genets increased while the number of genets and production of sporocarps per unit area decreased. There was an estimated 700–5700 genets ha−1 in younger forests and 30–120 ha−1 in older ones. The maximum size of genets was 1.7–5.3 m in the younger forest and to 17.5 m in the older ones. The production of sporocarps per unit area decreased with increasing forest age. Furthermore, production of sporocarps increased nonlinearly with the size of the genet, suggesting that genets become fragmented. Ergosterol measurements indicated that the fungal biomass of one genet consisted of 20–45% sporocarps and 55–80% mycorrhiza, not including extramatrical mycelia. Mycorrhizal aggregations in soil were mapped in two 5 m2 areas and, based on somatic incompatibility tests, all were round CO belong to the same genet as sporocarps present above ground. Production of spores per sporocarp was estimated to be 1.1–12.8 × 108. The observations are discussed in terms of population ecology.