Arbuscular mycorrhizal (AM) fungi, which have often been designated as vesicular-arbuscular mycorrhizal (VAM) fungi, symbiotically inhabit plant roots. The promotion of phosphorus uptake and plant growth of host by AM fungi is now well recognized. Therefore, it is anticipated that AM symbiosis could be applied for sustainable agriculture. The author investigated the regulation of the symbiosis with emphasis placed on nutrient exchange, namely phosphorus transfer from AM fungi to host plant and reverse transfer of carbon. The following results were obtained: 1) The growth of AM fungi in host roots was regulated by the supply of photosynthates and the growth of both partners in the symbiosis was well harmonized. 2) A new method for the isolation of intraradical hyphae from host root tissues was developed. The characteristics of enzyme activities in the isolated intaradical hyphae suggest that alkaline phosphatase activity is localized in arbuscular hyphae and that glucose is one of the carbon sources from host plant to AM fungi. 3) Electrophoresis analysis showed that symbiosis-specific may originate from AM fungi. Based on these findings, the regulation of the symbiosis was discussed.