Regions with Mediterranean climates (i.e., with winter rainfall and summer drought) have many different agricultural systems. However, two common features of these regions are the use of rotations of legumes and cereals, and the relatively low levels of use of phosphate fertilizer applied to phosphate-deficient soils. There is potential to increase the efficiency of use of phosphate in these environments by increasing the contribution of vesicular arbuscular (VA) mycorrhizal fungi because legumes respond to mycorrhizal colonization when phosphate is limiting plant growth. The potential to make effective use of the mycorrhizal symbiosis in Mediterranean agriculture depends on developing further understanding of several key relationships between the components of the symbiosis and the soil conditions. First, there is a need to know whether the existing level of colonization of roots by mycorrhizal fungi is less than optimal. To achieve this, a combination of an understanding of the effects of agricultural practices (as they affect the dynamics of populations of the fungi in soil) and a calibrated bioassay to predict the level of colonization of roots in the following season are required. The second relationship required is the interaction between the rate and extent of colonization of roots and the functioning of the mycorrhizas. The impact of agricultural practices on the functioning of the symbiosis also needs to be included in the analysis.