Bacterial blight caused by Xanthomonas oryzaepv oryzae is a devastating disease in rice worldwide.The resistance gene Xa4 has been widely used in breedingprograms and played an important role in protectingrice from this disease. Using 642 highly susceptible individualsand a random sample of 255 individuals from anF2 population developed from a cross between IRBB4and IR24, the Xa4 gene was genetically mapped to aregion less than 1 cM. A contig map was constructed forthe Xa4 region consisting of six non-redundant bacterialartificial chromosome (BAC) clones and spanning approximately500 kb in length. Analysis of recombinationevents in the Xa4 region located the gene locus to oneBAC, 3H8. Assay of the recombinants using the subclonesof 3H8 in combination with sequence analysisfurther narrowed the Xa4 locus down to a 47-kb fragment.