Plant flavonoids reported previously to act as molecular signals in the arbuscular mycorrhizal (AM) symbiosis are known to bind to estrogen receptors and to exert estrogenic effects on mammalian cells. To further investigate the estrogen-like properties of flavonoids the present study examined whether estrogen and antiestrogens have fla-vonoid-related functions in AM fungi. Bioassays were performed in a monoaxenic system with the AM fungi Giga-spora margarita and Glomus intraradices. The plant flavo-noids quercetin and biochanin A stimulate hyphal growth of G. margarita and G. intraradices, respectively. The stimulatory activity of biochanin A studied at concentrations ranging from 0.01 to 10.0 μM shows an estimated EC50 value of 3.26 μM. The present results show that 17β-estradiol and biochanin A exert similar stimulatory effects in G. intraradices. The agonist effect of biochanin A was efficiently suppressed by the new antiestrogen EM-652, which is also consistent with the possible presence of estrogen-binding sites in AM fungi.