The concept of soil infectivity, widely used for soil-borne plant pathogens, is applied to the fungi forming vesicular–arbuscular endomycorrhizae. The authors propose a method for the determination of the mycorrhizal soil infectivity (MSI) using leek, a highly mycotrophic plant, for the bioassays. Under controlled conditions, populations of leek plants are grown in a range of concentrations of a natural soil mixed with autoclaved volumes of the same soil. The relationship between the percentage of plants forming mycorrhizae and the soil concentration is used as the basis for the determination of the quantity of soil required to obtain mycorrhiza formation on 50% of the host plant population; the results are defined in terms of MSI units and are expressed as MSI50 for 100 g of soil. The features of the method are illustrated in a comparative study of four agricultural soils from France.