Phytohormones are thought to mediate plant-arbuscular mycorrhizal (AM) interactions. To explore the role of phytohormones in the interaction between Nicotiana attenuata and Glomus intraradices, we analysed levels of jasmonic acid (JA) and its amino acid conjugate JA-isoleucine/JA-leucine (JA-Ile), salicylic acid (SA) and ethylene in either infected or non-infected N. attenuata wild-type (WT) plants growing in soils that mimic the nutrient supply rates found in the plant's native environment. Under these conditions, the infection decreases plant growth and reproductive performance. Levels of JA, JA-Ile and SA did not change upon infection, but ethylene release was slightly decreased. Transgenic N. attenuata plants defective in JA signalling (aslox3 and ircoi1) did not differ significantly in growth or reproductive performance compared with infected WT. Furthermore, no difference in infection rates could be observed. Transgenic plants unable to produce (iraco) or perceive (etr1) ethylene showed significantly larger decreases in growth and number of seed capsules produced between infected and non-infected plants compared with WT plants. We conclude that ethylene, rather than JA, signalling plays a role in the interaction between N. attenuata and AM, from which the plant does not realize a fitness benefit.