The main host carbon energy source transferred from wheat leaves (Triticum aestivum L.) to wheat powdery mildew (Erysiphe graminis f.sp. tritici) has been investigated in three ways. When the uptake of sugars by isolated mycelial suspensions was examined, the uptake rate for glucose was considerably higher than that for a range of other solutes. Analysis by high-performance liquid chromatography of leaf and mycelial extracts following uptake of sugars into infected leaf pieces confirmed that sucrose was rapidly hydrolyzed in the leaf; no sucrose or fructose could be detected in mycelial extracts. Furthermore, studies of the uptake of asymmetrically labelled sucrose indicated that this sugar is cleaved prior to uptake by the pathogen. Thus several lines of evidence show that glucose, and not sucrose, is the major carbon energy source transferred from host to fungal mycelium.