Vesicular-arbuscular (VA) mycorrhizas are widely distributed but there is limited knowledge of the occurrence of individual species in relation to soil, climate and vegetation. Recent developments in methods for identifying and quantifying VAM fungi, both in soil and within roots, should enable the clearer definition of the factors that influence their distribution. Careful attention must be paid to sampling procedures to avoid bias and subjectivity. Soils commonly contain more than one VAM fungus. The development of VA mycorrhizas varies with soil type and depth, season and vegetation. The dynamics of root colonization by individual species within a population have not been adequately explored. Minor or gradual disturbances in agricultural and natural ecosystems may, or may not, lead to marked changes in mycorrhiza formation. Populations of VAM fungi appear to be capable of adjusting to gradual changes in the environment without abrupt changes in the extent of colonization. In contrast, more extreme, or rapid environmental changes such as those associated with mining or erosion, may markedly decrease mycorrhiza formation. The restoration of populations of mycorrhizal fungi will depend on the availability of accessible sources of propagules and on the suitability of the disturbed soils for plant and fungal growth.