Phloem sap from wheat seedlings differing in hydroxamic acid (Hx) concentrations was collected using aphid stylets excised by microcautery. DIMBOA glucoside, the major Hx in wheat seedlings, was found in phloem sap in concentrations which did not differ between cultivars. Using electrical penetration graphs, the feeding behavior of the cereal aphid Rhopalosiphum padi (L.) was studied in wheat seedlings and also in artificial diets differing in Hx concentrations. Linear correlations were found between: i) Hx concentrations in whole seedlings and time taken by aphids to engage in a committed phloem ingestion, and ii) DIMBOA concentration in diets and ingestion times in them. Mean committed phloem ingestion times were not significantly different between the cultivars studied. These results point to a feeding deterrency by DIMBOA in the plant’s mesophyll tissue.