The mycorrhizal fungus Boletus variegatus Fr. was grown in pure culture and its production of volatile organic compounds studied. Maximal production of volatile organic compounds was associated with actively growing mycelia. The major volatile compounds identified in the culture filtrate were ethanol, isobutanol, isoamyl alcohol, acetoin, and isobutyric acid. Of these, isobutanol and isobutyric acid are known to possess fungistatic activity. Volatile organic compounds were also extracted from the whole root systems of Pinus sylvestris L. (Scots pine) seedlings grown in pure culture with and without the fungal symbiont. Infection of the roots by the fungus resulted in production and (or) accumulation of volatile compounds in concentrations two to eight times greater than that of non-inoculated controls. These compounds were identified by combined gas chromatography and mass spectrometry. They were primarily terpenes and sesquiterpenes. Volatile compounds produced by the mycorrhizal root system of Scots pine collected from a nursery were essentially the same as those obtained from the plants grown in pure culture. Many of these are known to be fungistatic compounds. A hypothesis has been proposed to explain a possible role of the host plant in disease resistance of mycorrhizal root systems to root pathogens and in the development of the symbiotic state.