Xanthomonas oryzaepv.oryzaeis the causal agent of bacterial leaf blight, a serious disease of rice. Spontaneous loss of virulence inX.oryzaepv.oryzaeis associated with mutants that produce reduced levels of extracellular polysaccharide. Evidence is presented that these mutants accumulate in long-term stationary phase cultures but are not obtained from exponentially growing cultures. A number of independentspv(stationary-phase variation) mutants have been isolated and characterized. These mutants differ from each other with respect to the stability of the mutant phenotype, extent of loss of virulence, ability to outcompete wild type cells in stationary-phase cocultures, and hypersensitivity to hydrogen peroxide.Key words: stationary-phase variation, spontaneous loss of virulence,Xanthomonas oryzae.