The influence of vesicular–arbuscular mycorrhizal (M) colonization on biomass production and photosynthesis of Trifolium repens L. was investigated in two experiments in which the foliar nitrogen and phosphorus contents of non-mycorrhizal (NM) plants were manipulated to be no lower than that of M plants. Throughout both experiments there was a stimulation in the rate of CO2 assimilation of the youngest, fully expanded leaf of M compared with NM plants. In addition, M plants exhibited a higher specific leaf area compared with NM plants, a response that maximized the area available for CO2 assimilation per unit of carbon (C) invested. Despite the increased rate of photosynthesis in M plants there was no evidence that the additional C gained was converted to biomass production of M plants. It is suggested that this additional C gained by colonized plants was allocated to the mycorrhizal fungus and that it is the fungus, by acting as a sink for assimilates, that facilitated the stimulation in the rate of photosynthesis of the plant partner.