Plant phenolic compounds are known to be inducers of virulence genes in plant-pathogen interactions such as those involving Agrobacterium, and flavonoids are known to be inducers or inhibitors of Nod genes in Rhizobium-legume symbiosis. More recent studies suggest that some of these compounds act as molecular signals in the development of vesicular-arbuscular mycorrhizas (VAM). The present study has shown that hyphal growth of the VAM fungus, Gigaspora margarita Becker & Hall, is affected by both stimulatory and inhibitory flavonoids, when applied at 10 μ together with an optimal carbon dioxide enrichment. Stimulatory compounds were all flavonols (kaempferol, quercetin and morin) and possessed at least one hydroxyl group on the B ring. Conversely, two isoflavones (biochanin A, and genistein), a single flavanone (hesperetin) and two compounds without any hydroxyl group on the B ring, galangin (flavonol) and chrysin (flavone), were all inhibitors of hyphal growth.