Polyphosphates have been extracted from onion roots infected with Glomus mosseae by the technique of phenol-detergent extraction. The polyphosphate was separated from RNA and DNA and characterized by the technique of polyacrylamide gel electrophoresis. On gels stained with Toluidine Blue, the blue-purple nucleic acid bands (β-metachromatic) could be readily distinguished from the pink (γ-metachromatic) polyphosphate band, which ran faster than tRNA, and was coincident with synthetic, marker polyphosphate of chain length 200 ±10. By excising stained gel bands, dry-ashing and P determination, and by enzymic hydrolysis, the presence of condensed orthophosphate in this band has been confirmed. Polyphosphates could not be detected in uninfected roots and the content in infected roots increases several-fold when plants are supplied with orthophosphate 24 h before extraction. By calculation it has been shown that the polyphosphate forms a substantial proportion (40%) of the total P present in the fungal component of mycorrhizal roots, and that this concentration is sufficient to support the proposed phosphorus translocation.