The ability of the Frankia endophyte to infect its host plant may be markedly altered by changes in soil conditions. Plant bioassays showed lowering soil water potential caused significant declines in plant infection rates. Organic matter levels also influenced infection with soils containing zero and high levels of organic matter exhibiting the greatest root nodule formation. The highest infection rates of Frankia occurred at pH 6.0, although these levels were not significantly different within the range of the test. Addition of combined nitrogen to soils had a substantial positive effect on infection rates of Frankia endophyte;Presence of the actinorhizal host plant, or any plant cover, were unnecessary for maintaining viable, infective populations of the Frankia endophyte;Hyphal growth of Frankia occurred outside host plant root nodules under sterile soil conditions. No evidence of hyphal growth was found under natural soil conditions as measured by Fluorescent Antibody (FA) or Enzyme Linked Immunosorbent (ELISA) assays, however. Results suggest that the spore form of Frankia is predominant under natural conditions.