The occurrence and the mechanisms of the transmission of Pasteurella tularensis from infected animals to healthy normal or vaccinated cage mates ("contagion-infection") was investigated. Fifty-seven (23%) of 249 normal mice and 9 (4%) of 204 vaccinated mice succumbed after exposure to infected mice. In the majority of the cases (64% of the normal and 67% of the vaccinated mice that died) the deaths occurred during the standard 14-day period of observation. Normal mice caged with infected mice may die as early as the 8th day; deaths occurred as late as the 14th day among singly-caged mice injected with minimal doses of P. tularensis. Thus, the period of observation cannot be shortened to avoid contagion-infection. Among the mechanisms by which this phenomenon may be produced are cannibalism of infected carcasses and air-borne infection. We have been unable to confirm the observation of Francis and Lake that the mouse louse (Polyplax serrata) is an efficient vector. Contagion-infection with P. tularensis can occur in guinea pigs, and normal mice may experience infections similarly contracted from mice inoculated with P. pestis.