AbstractLeaf segments from seedlings of 68 old barley varieties and 38 more recent cultivars of past or current importance in France were infected with 6—9 powdery mildew isolates in order to identify race‐specific resistance genes. No resistance gene was apparent in the 21 old winter varieties, 43 of 47 old spring varieties, 10 recent winter and 3 recent spring cultivars. Two old spring varieties (‘Colmar S 142’ and ‘Johanna’) were postulated to haveMlgandMl(CP), and two others (‘Pontrieux’ and ‘Finistère 62‐5’) had an unidentified, weakly effective gene.Mlg, Ml(CP), Mlb, Mla6andMlal3were the only genes detected in recent winter cultivars. Recent spring cultivars presented the greatest diversity; the presence of one or several genes amongMlal, Mla6, Mla9, Mla12, Mlg, Ml(CP), Ml(La), Mlkandmlowas postu