The importance of interactions between arbuscular-mycorrhizal fungi (AMF) and weeds of agroecosystems is reviewed. Considerable evidence suggests that AMF can aect the nature of weed communities in agro-ecosystems in a variety of ways, including changing the relative abundance of mycotrophic weed species (hosts of AMF), and non-mycotrophic species (non-hosts). These eects may merely change the composition of weed communities without aecting the damage that these communities cause. However, it is quite plausible that interactions with AMF can increase the bene®cial eects of weeds on the functioning of agro-ecosystems. Through a variety of mechanisms, weed:AMF interactions may reduce crop yield losses to weeds, limit weed species shifts, and increase positive eects of weeds on soil quality and bene®cial organisms. If bene®cial eects of AMF on the composition and functioning of weed communities can be con®rmed by more direct evidence, then AMF could provide a new means of ecologically-based weed management. Intentional management will be required to increase diversity and abundance of AMF in many cropping systems, but these actions (e.g. conservation tillage and use of cover and green-manure crops) typically will confer a range of agronomic bene®ts in addition to potential improvements in weed management 1 .AMF effects on weed functional ecology: nutrition, seed germination, pathogen resistance and stress tolerance AMF colonize roots of`mycotrophic' plant species (`host' species hereafter) and form mycorrhizae, which are intimate connections between fungus and plant root. Mycorrhizae are 398 N R Jordan et al.