1. Three annual plant species Abutilon theophrasti Amaranthus retroflexus and Setaria lutescens were grown together in field plots at two different phosphorus levels and in either fumigated soil fumigated soil which was subsequently re- inoculated with spores of the mycorrhizal fungus Glomus intraradices or untreated soil containing indigenous mycorrhizal fungi. 2. The response of each plant species to the mycorrhizal treatments differed significantly. The results indicated that mycorrhizal fungi reduced the growth of Amaranthus (non-mycotrophic) and increased the growth of Abutilon (mycotrophic). Mycorrhizal treatment had little effect on the performance of Setaria (mycotrophic). 3. Significantly higher concentrations of phosphorus in mature reproductive parts of both mycotrophic species in treatments where mycorrhizal fungi were present suggest that the mycorrhizal symbiosis could significantly affect the quality of seed and may have long-term effects on the structure of plant communities.