Technical Abstract: A diverse collection of modern, heirloom and specialty cultivars, Plant Introduction (PI) accessions, and breeding lines of lettuce were screened for susceptibility to lettuce dieback, which is a disease caused by soilborne viruses of the family Tombusviridae. Of the 241 genotypes tested in field experiments, 76 remained symptom-free in infested fields and were therefore classified as resistant to dieback. Overall, resistance genotypes were as prevalent in modern cultivars as in heirloom or primitive germplasm. Within modern germplasm, however, crisphead (iceberg) cultivars were uniformly resistant, while romaine cultivars were uniformly susceptible. Using enzyme-linked immunosorbent assay (ELISA), tombusviruses were detected in leaves of some resistant plants that were grown in infested fields or that were inoculated as seedlings in the growth chamber, suggesting that symptom-free plants are not immune to viral infection. The inheritance of resistance was studied for both 'Salinas', a modern crisphead cultivar, and PI 491224, the progenitor of recently released romaine germplasm with resistance to lettuce dieback. Data showed that resistance was conferred by a dominant allele at a single locus in both genotypes. The tombusvirus resistance locus from 'Salinas', Tvrl, was mapped in an intraspecific L. sativa population to a location that corresponds to linkage group 2 on the consensus map of Lactuca. The largest cluster of resistance genes in lettuce, the Dm1/Dm3 cluster, is found on this linkage group, however, the precise position of Tvrl relative to this cluster has not yet been determined. To our knowledge, Tvrl is the first Tombusvirus resistance gene identified for any plant host.