Colonization of roots by arbuscular mycorrhizal (AM) fungi in several annual crops in two consecutive seasons was compared with, in the second season, the density of fungal propagules in the soil with the use of a bioassay. Root density decreased down the soil profile in both years in all crops, and a high proportion of roots were mycorrhizal throughout the profile. AM colonization decreased down the profile in cotton and lablab in the second season only. The bioassay indicated that most propagules of AM fungi in soils under cotton were located near the surface, with virtually no propagules at 1 m. The absence of propagules at depth indicates a lack of mycelium deep in the soil, and suggests that mycorrhizas are primarily initiated in the surface soil and that the fungi colonize the root system mostly through secondary spread down the profile. The use of AM colonization in the field as an indicator of propagule density and symbiotic function should be qualified by an understanding of the depth in the soil from which roots were extracted.