Seedlings and adults of Hypericum perforatum L., common St. John's wort, were grown together in an experiment of factorial design, where the presence of root competition and arbuscular mycorrhizal inoculation were used as treatments. There was moderate shoot competition, the intensity of which was not manipulated. To check the response of plants to arbuscular mycorrhiza in noncompetitive conditions, adults and seedlings were also grown singly in pots. Single individuals of seedlings and adults responded positively to mycorrhizal inoculation, with the response of seedlings significantly greater. In the competition experiment the positive effect of mycorrhizal colonization on seedling growth vanished, since with root competition, both mycorrhizal and nonmycorrhizal seedlings were of the same size. Without root competition, the shoots of mycorrhizal adults were much larger than of nonmycorrhizal adults, but under root competition the shoot weight did not differ. Arbuscular mycorrhiza increased the biomass differences between competing seedlings and adults. We did not confirm our hypothesis that mycorrhiza makes competition between seedlings and adults more balanced because of the stronger positive response of seedlings to inoculation. The positive effect of arbuscular mycorrhizae on growth vanishes in more crowded conditions. It was concluded that if mycorrhizal inoculation has age-specific positive effect of seedlings in field conditions, it is more probably due to higher tolerance to abiotic stress than due to higher competitive ability of seedlings.Key words: arbuscular mycorrhiza, intraspecific root competition, seedlings, Hypericum perforatum.