Arbuscular mycorrhizal (AM) mycelia are dependent on contact with plant roots for spore formation. In this study, earthworms and collembolans were regarded as potential dispersal vectors of AM fungal spores and hyphae, and we determined how they influenced the extension of AM fungi from host plant roots. Plantago lanceolata seedlings were grown in a mesh bag with an AM inoculurn of dried soil and root pieces from a set-aside agricultural field. The bag was placed in a growth box that was filled with a mixture of irradiated soil and sand, into which fungal hyphae could grow while roots were retained by the mesh. Three treatments with 15 replicates each were established with an addition of either Lumbricus rubellus, Folsomia candida or no soil fauna into the root-free soil. Five replicates of each treatment were harvested after 6, 10 and 15 weeks. The extension of the AM fungi was determined by measuring PLFA 16:1w5 and NLFA 16:1w5 as signature compounds of AM fungal biomass, spore production and mycorrhizal inoculum potential on P lanceolata seedlings. After 10 weeks, all indicators of AM fungal growth showed that the fungi had extended into the whole growth box. Plant shoot biomass of the host plants was greater in the presence of earthworms than other treatments, and the contents of PLFA 16:1w5 were greater in the earthworm than in the collembolan treatment. The reduced amount of PLFA 18:2w6,9 in the collembolan treatment relative to other treatments suggests that saprophytic fungi provided a food source for the collembolans. Spore production and mycorrhizal inoculurn potential were not affected by the presence of soil fauna. We conclude that the biomass of AM fungi was stimulated by earthworms, but that fungal dispersal over 20 cm was not influenced. (C) 2004 Elsevier B.V. All rights reserved.