ABSTRACTDifferent forms of vesicular-arbuscular mycorrhizal fungal inoculum are found in soil patches with differing histories. These inoculum types include spores, infected root fragments, and extramatrical hyphae. Utilizing root observation chambers, detailed observations were made of the growth dynamics of mycorrhizal hyphae from inoculum sources to roots and the subsequent radiation out from individual colonized root segments. There were several types of hyphal architecture classified by whether the external hyphae were entering (infecting) a plant root or developing from the root surface and growing out into the soil matrix. Morphologically, individual hyphal filaments within all architectural types looked quite similar and clear architectural differences could only be identified when distinct sets of interconnected hyphal filaments were studied. The primary hyphal architectural types observed in the soil were classified as runner hyphae, hyphal bridges, absorptive hyphal networks, germ tubes, and infection networks produced by spores and root fragments. The specialized hyphal architectures of these fungal endophytes appear to be linked to the unique function of each hyphal type. Runner hyphae and infection networks from either spores or root fragments were observed to be capable of infecting new root segments. Absorptive hyphal networks were never observed to act as units of infection and were classified as structures primarily involved in the acquisition of soil resources. This high degree of specialization may help explain why vesicular-arbuscular mycorrhizal fungi are so efficient in their beneficial role of nutrient and water transport to the host plant.