Abstract In ectomycorrhizas, mutual benefit between the plant and fungal partners is due to the exchange of plant-derived carbohydrates for amino acids and nutrients supplied by the fungus. Sucrose, the major plant transport carbohydrate, is hydrolysed in the common apoplastic interface, and monosaccharides are taken up by both the fungal partner and root cortical cells. The plant partner reduces its import capacity for monosaccharides in the symbiosis, presumably due to its supply of fungus-derived amino acids. In contrast, the expression of an Amanita muscaria monosaccharide transporter (there is evidence that it is the only one in this fungus) was enhanced approx. 4–6-fold in ectomycorrhizas compared to soil growing hyphae, leading to a significant increase of the fungal monosaccharide import capacity in symbiosis. This strong carbohydrate sink might explain why, in ectomycorrhizas, up to 30% of the total photoassimilate production is transferred to the fungal partner. The plant on the other hand compensates for this carbohydrate loss by increasing its photosynthetic efficiency. Monosaccharides are not only important for fungal growth and function, but also constitute signal molecules that regulate fungal and presumably also plant gene expression in symbiosis. Two types of sugar-controlled gene regulation could be distinguished in the ectomycorrhizal fungus A. muscaria. The first is gene repression. The expression of a phenylalanine ammonia lyase is strongly repressed by external hexoses. This type of monosaccharide-dependent control is possibly regulated by hexokinase as a sensor. The second is gene induction. The expression of an A. muscaria monosaccharide transporter is increased when the external hexose concentration exceeds 5 m M . The sensor as well as the mechanism of this sugar-regulated induction of gene expression is still unknown.