Several years ago we reported the occurrence of potyvirus infecting maize in southern France and in Spain and Italy, the most prevalent virus being maize dwarf mosaic virus (MDMV) strain A. MDMV is present on dent and sweet corn mainly in areas where Johnson grass (Sorghum halepense L), the reservoir host, is abundant. The aim of our work was to obtain a good and reliable diagnostic method and to start to study the biodiversity of MDMV. We have obtained a good specific antiserum directed against the capsid protein of the virus. In 1991, we started a collection now made up of more than 100 isolates from southern France, the northern part of Italy and some areas of northern Spain. The molecular weights of the capsid protein of all the isolates were determined by the immunoblot method. We have been able to classify the isolates in different groups. The behaviour of the isolates belonging to each group defined by Western blot was analysed by mechanical inoculation on several maize inbred lines. We have thus demonstrated a biodiversity in MDMV. This situation is important for the breeder and allows a better understanding of the behaviour of inbred lines of maize in different areas.