Aeromonas hydrophila, an opportunist human pathogen of low virulence, was shown to display a high degree of sensitivity upon exposure to hydrogen peroxide. As with other species, Aer. hydrophila is able to develop the capacity to resist loss of viability induced by such oxidative stress. Development of stress resistance follows the archetypal pro®le where pre-exposure of a population to sub-lethal levels of H 2 O 2 stimulates onset of tolerance to further exposure. Acquisition of tolerance critically requires nascent protein synthesis. Further analysis demonstrated population growth phase in¯uences the degree of sensitivity of the organism.Late stationary phase cultures demonstrate a decreased sensitivity compared with younger populations. Signi®cantly, it was also determined that stock culture age in¯uenced the level of sensitivity of the derived experimental culture, where an increased stock culture age corresponded with enhanced resistance to H 2 O 2 . These data show that Aer. hydrophila population phenotype is in¯uenced by the phenotype of the donor stock culture.