To investigate the effect of low light intensities and the inoculation of Frankia and/or Alpova diplophloeus on the symbioses development and their host growth, red alder(Alnus rubra Bong.) seedlings were grown in an air - filtered walk - in growth chamber with either N₂ - fixing Frankia inoculation or N - fertilization and live or dead spore inoculation of the ectomycorrhizal fungus: A. diplophloeus(Zeller & Dodge) Trappe & Smith. When they were 20 weeks old, the seedlings were grown under three levels of light intensities of 680, 320 and 220μ㏖/㎡/s PPFD(photosynthetic photon flux density) for three weeks. PPFD of 220 significantly decreased the development of A. aiplophloeus mycorrhizae and nodules, the rates of N₂ - fixation and CO₂ exchange, and the growth of tile seedlings. PPFD 320 significantly decreased the CO₂ exchange rate only. Frankia inoculation significantly increased mycorrhiza formation and seedling growth. Alpoaa inoculation significantly increased seedling growth but not nodule development and N₂- fixation. None of the symbionts affected CO₂ exchange rates. Frankia was more critical for seedling growth and mycorrhizal development than the mycorrhizal fungus for seedling growth and nodule development.