Plants of Pinus spp. were grown in observation chambers with the mycorrhizal fungi Suillus bovinus, Pisolithus tinctorius or Paxillus involutus. After interconnecting mycelial systems had developed between plants, individual hosts in some chambers of each species were fed with 14CO2. Mycelia from radioactively labelled and unlabelled chambers were harvested and their carbohydrates were extracted, separated chromatographically and identified. The major carbohydrates in all of the fungi were trehalose, mannitol and arabitol, their relative proportions differing in the different fungi. The results are discussed in relation both to carbon nutrition of the fungus and to carbon transfer between interconnected plants.