Sucrose-agarose gels and sucrose liquid diets were used to study the phenol oxidising enzymes in the salivary secretions of the grain aphid, Sitobion avenae (Fabricius). Activity indicating the presence of two oxidoreductases, polyphenol oxidase (PPO) and peroxidase (Px), was found. Both enzymes were present in the aphid's stylet sheath (gelling saliva) but only polyphenol oxidase activity was found in the halos around sheaths and thus in watery saliva. Electrical penetration graphs (EPG) revealed that the secretion of these enzymes into the gels, by an individual aphid, was associated with its probing activity observed during penetration of the epidermal and mesophyll tissues. The grain aphid's PPO, secreted in its saliva reacted with a range of phenolic compounds. As most of these phenolics occur naturally in cereals, the grain aphid could modify its host-plant's phenolic composition. The importance of the grain aphid's polyphenol oxidase and peroxidase in detoxifying cereal phenolics is discussed.