The genetics of resistance to bacterial blight, Xanthomonas oryzae (Uyeda and Ishiyama) Dowson, for 74 cultivars of rice, Oryza sativa L., was studied. The PX061 isolate of bacterial blight from the Philippines was used for inoculation of parental and hybrid populations. Single dominant genes at the Xa 4 locus convey resistance in 38 cultivars. Of these, 18 are resistant at all stages of plant growth and thus have the Xa 4 (a)allele for resistance. However, 20 are susceptible up to maximum tillering stage but are resistant at booting and flowering stages. These cultivars have the Xa 4 (b)allele for resistance. Thirty-two cultivars have single recessive genes for resistance which are allelic to xa 5.The resistance in 'DV85', 'DV86' and 'DZ78' is conditioned by two genes. At maximum tillering stage xa 5 conveys resistance. However, at later growth stages an additional dominant gene, designated Xa 7 in 'DZ78', also gives resistance. The dominant genes of 'DV85' and 'DV86' are probably allelic to Xa 7. Xa 7 segregates independently of Xa 4, xa 5 and Xa 6, however like Xa 6, it conveys resistance at booting and post-booting stages only.The resistance in 'PI 231129' is conditioned by a single recessive gene, designated xa 8. It also segregates independently of Xa 4, xa 5 and Xa 6.