The presence of several classes of reducing substances in mycorrhizal roots of beech is demonstrated, throwing considerable doubt on the values of ‘easily soluble reducing substances’ which have been used for correlation with degree of mycorrhizal infection by several workers. The soluble sugars of beech mycorrhizas are glucose, fructose, sucrose and trehalose, together with the acyclic polyol, mannitol and two cyclic polyols, myo-inositol and an unidentified inositol. Trehalose and mannitol are absent from uninfected roots showing their presence to be dependent on fungal infection. Methods for the individual assay of the soluble carbohydrates are presented. The main sugars obtained by acid hydrolysis of the insoluble residue of mycorrhizas are galactose, glucose, mannose, arabinose, xylose, fucose, ribose, rhamnose and a deoxypentose provisionally identified as deoxyribose. The mannose and deoxypentose polymers are absent from beech tissue and deoxyhexose polymers from the fungus. Much of the glucose is derived from fungal glycogen. Mycorrhizas accumulate principally trehalose and glycogen from exogenous glucose and trehalose, and mannitol from fructose and mannitol. The pattern of synthesis from exogenous sucrose is intermediate between glucose and fructose. The results are compared with those obtained for other fungi by other workers and a possible ecologically significant role for mannitol is suggested.