Germin‐like oxalate oxidase is an oligomeric enzyme which generates H₂O₂. This paper reports increased activity of this enzyme in association with the response of barley to the powdery mildew fungus, Erysiphe graminis f.sp. hordei. The increase is detected in a colorimetric assay as well as on activity blots using extracts of both resistant and susceptible leaves. The increase is generally apparent from 24 h after inoculation. From 48 h after inoculation, there is an approximately 10‐fold higher oxalate oxidase activity in the samples from inoculated plants compared with the controls. The oxalate oxidase activity increase appears 1–3 days earlier than PR‐1 protein accumulation. SDS—PAGE analysis suggests that this pathogen‐response oxalate oxidase is different from a commercially available barley oxalate oxidase, and from a constitutive barley root oxalate oxidase.