Restriction fragment length polymorphism (RFLP) analysis was used to identify a major gene for symptom response (resistance) to MDMV strain A (MDMV-A) on chromosome 6 of the maize imbred Pa405. RFLP analysis of individual backcross plants of the genotypes (pa405 .times. yM14) .times. yM14 and (Pa405 .times. K55) .times. K55, inoculated with MDMV-A, mapped this resistance gene to a region near the centromere of chromosome 6. This gene is tightly linked to and liked between the RFLP marker loci, UMC85 and BNL6.29. This gene is essential for any resistance response because all plants lacking this gene rapidly developed generalized mosaic symptoms. In yM14 backcross plants, this gene both delayed symptom appearance and reduced symptom severity. In K55 backcross plants, this gene caused a similar delay in symptom appearance; however, the delayed symptoms were more variable and more severe than in the yM14 backcross plants, indicating that the effect of this gene on symptoms development can be influenced by the genotypes of the susceptible parents. We proposed to designate this gene Mdm1 for maize dwarf mosaic virus resistance, gene 1.