Tomato yellow leaf curl virus (TYLCV), a Begomovirus in the family Geminiviridae, is an important disease of cultivated tomato (Solanum lycopersicum L.) in many parts of the world. Disease is managed primarily by chemical control of the vector, the sweetpotato whitefly, Bemisia tabaci (Genn.), and by growing resistant varieties. Resistance derived from the cultivar Tyking is being used in many breeding programs, but the location of resistance factors has not been reported. The breeding lines Fla. 8753 and Fla. 344 both have high levels of resistance to TYLCV derived from ‘Tyking’ and from S. chilense accession LA 1938, but none of their parent lines contain any of the known genes Ty-1 to Ty-4. An additional resistance locus, Ty-5, was recently identified, and to determine if this locus controls TYLCV resistance in Fla. 8753 and Fla. 344, appropriate segregating populations were analyzed using the Ty-5 marker, SlNAC1. Results show that SlNAC1 cosegregates with a recessive allele derived from ‘Tyking’. We suggest the gene symbol ty-5 be used to describe this gene. Mean disease severity of progeny homozygous for either the resistant or susceptible alleles did not equal parental levels of resistance and susceptibility, respectively, suggesting the involvement of an additional gene that is likely derived from LA1938.