Technical Abstract: Corky root is a major disease of lettuce (Lactuca sativa L.) observed in most production areas of the world. The pathogen Sphingomonas suberifaciens (formerly, Rhizomonas suberifaciens) varies with regard to virulence, and several strains have been isolated that can cause certain disease symptom on cultivars with the only known resistance gene, cor. It is desirable to find new sources of resistance to diversify the genetic basis of the resistance and to confer resistance to isolates that are not adequately controlled by cor. More than 1,000 plant introduction lines and cultivars were screened in the greenhouse, growth chamber, and field. Three L. serriola lines (PI 491239, PI 491096, and PI 491110) and a L. virosa line (PI 273597c) were highly resistant to corky root in all tests. Disease severity ratings in the field was correlated with the ratings in the greenhouse (r = 0.722) and in the growth chamber (r = 0.650). None of the four resistant lines had the two molecular markers closely linked to the cor allele. The information on disease resistance for these germplasm will be useful in future breeding work.