The effects of photon irradiance, phosphorus (P) supply and mycorrhizal infection [Glomus mosseae (Nicol. and Gerd) Gerdemann and Trappe] on growth and P uptake by Allium cepa L. were measured. Two experiments were carried out, one in a naturally-lit glasshouse and the other in a growth cabinet. Shade cloth was used to provide ‘high’ irradiance (600 and 550 μmol m−2s−1 in glasshouse and growth cabinet respectively) and ‘low’ irradiance (250 μmol m−2s−1). Growth at high irradiance was associated with increases in total dry weight, root: shoot dry weight ratio and total root length and with decreases in shoot fresh weight: dry weight ratio. Mycorrhizal growth responses were lower at low irradiance and depressions were observed at high P supply and high irradiance, as well as at low irradiance. Mycorrhizal plants generally had higher shoot fresh weight: dry weight ratios than those of equivalent non-mycorrhizal plants. Mycorrhizal infection was lower in the presence of additional P in both experiments. In the glasshouse there was an interaction between low irradiance and high P, leading to very low values of fractional infection. This effect was not apparent in the growth cabinet. At high irradiance mycorrhizal plants had higher inflows than those of equivalent non-mycorrhizal plants at all levels of P, regardless of the growth response. Low irradiance had little effect on inflow into non-mycorrhizal plants, but depressed inflow into mycorrhizal plants. This reduction in inflow can be attributed in part to lower levels of infection, but an effect on P uptake by hyphae or on transport between the symbionts is also likely.