Pseudomonas spp. isolates from Tuber borchii ascocarps, known to be able to produce phytoregulatory and biocontrol substances in pure culture, were used to perform studies on their possible physiological role in nature. Antimycotic activity was confirmed against fungal contaminants isolated from the ascocarps, suggesting that populations associated with Tuber borchii fruit bodies may play a role in the maintenance of ascocarp health. Fifty-five percent of strains tested were also able to release metabolites which affected T. borchii mycelial growth and morphogenesis in culture. On the contrary, growth of the arbuscular mycorrhizal fungus Glomus mosseae and the ectomycorrhizal fungus Laccaria bicolor, putative competitors of Tuber for mycorrhizal infection sites on roots, was not influenced by the presence of any bacterial strain. The possibility that these bacteria, which show antifungal activity and fungal growth modulation activities, might be incorporated in the developing ascocarp by means of their preferential adhesion to Tuber mycelium is discussed.