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The observation of Delwiche et al that the virulence of broth cultures of Pasteurella pestis aerated at 37 C can be maintained by addition of bicarbonate was confirmed and extended. A differential growth response by virulent and avirulent cells to added bicarbonate under certain conditions was demonstrated. The effect of supplemental bicarbonate on initiation of growth from a virulent inoculum was found to be (a) stimulation of both virulent and avirulent cells at low levels of added bicarbonate, (b) depression of both virulent and avirulent cells at higher initial concentrations, or (c) depression of avirulent mutants with simultaneous stimulation of virulent cells at a narrow range of intermediate levels.