The molecular and genetic control of the interaction between tomato races of Xanthomonas campestris pv. vesicatoria (XcvT) and tomato was studied. Based on inoculation phenotype and analysis of in planta bacterial growth, tomato line Hawaii 7998 is resistant to XcvT race 1 75-3 but not to XcvT race 2 89-1. Two cosmid clones from a genomic library of XcvT race 1 75-3 converted the normally virulent race 2 89-1 to avirulence on Hawaii 7998. The two clones contained the previously isolated, nonhost avirulence gene avrRxv, and their activity was localized to a 2.1-kbp subclone of avrRxv. avrRxv inhibits growth of race 2 89-1 in the resistant line Hawaii 7998 and an insertional mutation in avrRxv prevents this inhibition. In addition, a dramatic increase in electrolyte leakage of leaves of Hawaii 7998 occurred after 12-hr postinfiltration with race 2 89-1 carrying avrRxv. The nucleotide sequence of avrRxv revealed one major open reading frame (ORF) that accords well with activity analysis of nested deletions. ORF 2-2 encodes a putative protein of 374 amino acids with a molecular weight of 42.1 kDa and a pI of 10.7. Inheritance of the avrRxv-specific resistance in Hawaii 7998 was studied in a total of 587 F2 individuals from crosses between Hawaii 7998 and susceptible lines. The inheritance of avrRxv-specific resistance in Hawaii 7998 appears to be governed by more than one locus.