Abstract: [Objective] Ultrastructural alteration and hydrogen peroxide (H2O2) localization were examined in Xanthomonas under cellular injury using transmission electron microscopy. [Methods] Histochemical methods were used in the present study. [Results] Intriguingly, the injury led to presence of an additional location of H2O2 accumulation within the cells. There was an association between the frequency and size of the additional location of H2O2 accumulation and the degrees of injury. Furthermore, an additional ultrastructure, mesosomes, was also present in injured cells. The frequency and size of mesosomes also increased with the increasing degrees of injury. [Conclusion] Result of multiple linear regression showed that the size of mesosome plays as a key factor in the quantity of excess H2O2 accumulation in bacteria under cellular injury. Linear correlation was confirmed between quantity of excess H2O2 accumulation and the size of mesosome in injured cells. This finding intensely indicated that mesosomes are just the additional location of H2O2 accumulation in cells under cellular injury. The excess H2O2 accumulation in mesosomes should be a positive regulatory mechanism in bacteria under cellular injury.