In the Mediterranean countries, maize can be infected by several viruses, the most prevalent of which is maize dwarf mosaic virus strain A (MDMV-A). This potyvirus is non-persistently transmitted by several aphids and cannot be controlled by chemical insecticides. The reservoir host in the fields, Johnson grass (Sorghum halepense), is very common. Its control by herbicides is possible but not always feasible. In commercial hybrid maize fields, the incidence of MDMV is not very important but, in seed production inbred lines are severely affected and seed yield is very often decreased. The only method of control would be to release resistant inbred lines. In order to check the germplasm used in Europe we first started a study on the biodiversity of MDMV-A. The results obtained are presented in another paper; we can differentiate MDMV isolates according to the molecular weight of the capsid and the reaction of inoculated inbred lines. Mechanical inoculations of our germplasm using an artist’s airbrush have been carried out in climatic chambers and in the open field. To evaluate corn for resistance to MDMV-A in controlled conditions, we have used the disease index system proposed by Kuhn and Smith (1977) based on the percentage of plants developing symptoms and the date of symptom appearance after inoculation. In the field, we have made 10 observations concerning easily quantifiable characteristics throughout the growing period of maize. The index values were correlated to the incidence of disease (expressed as quantitative characteristics) for 650 inbred lines inoculated in the field.