Introduction Arbuscular mycorrhizal fungi (AMF) are the most ancient, widespread and ubiquitous of all the groups of mycorrhiza: they have a global distribution in widely contrasting plant communities including the Tropics, the Boreal forest, arctic tundra and all types of grassland. Considerable effort has been made in recent years in order to set AMF within a robust phylogeny. Recent advances in molecular biological techniques have enabled scientists to place AMF in a new division, the Glomeromycota. At present, this division contains only about 150 species, which is remarkable given the enormous number of plant species the fungi readily colonize. The mutualistic symbioses that AMF form with their host plants give rise to a number of important benefits to both the plant and fungus. A brief glance at a standard mycorrhizal text will list many ecologically important attributes, such as improved disease resistance, water uptake, nutrient transfer and the ability of the fungus to be a major sink for photosynthate. Indeed, the importance ofAMFfor nutrient uptake and carbon allocation has been recognized for decades. The ability of AMF (and other mycorrhizal types) to utilize recent plant photosynthate and thus have access to a near continuous supply of energy immediately gives them a potential advantage over saprotrophic microorganisms, which are forced to obtain their energy in the highly carbonlimited heterogeneous soil environment.