The cloned rice gene, Xa21, confers resistance to multiple pathogen isolates of Xanthomonas oryzae pv. oryzae in transgenic plants. The resistance phenotype was stably transmitted to T1 progeny and inherited as a single locus. The T1 progeny were tested for resistance to 32 X. oryzae pv. oryzae isolates from eight countries. Both the engineered line and the donor line showed resistance to 29 isolates and susceptibility to three isolates. The identical resistance spectrum of both lines indicates that the presence of a single member of a multigene family, Xa21, is sufficient to confer multi-isolate resistance. The results presented here have important implications for engineering disease resistance in crop plants.