Twenty-four monkeys were exposed to Pasteurella pestis strain 139L administered by the inhalatory route. Dosage ranged from approximately 24,2000 to approximately 112,000 organisms per monkey. Investigation of the cause of the low mortality rate (3 deaths, 21 survivors) led to the finding that survivors bore lesions which indicated a chronic pneumonic process in contrast to the acute character of the lesions seen in non-survivors. Virulent, phage-sensitive P. pestis was recovered from the survivors in two instances, one at 15 days, the other at 48 days following exposure to the infectious clouds. In additional trials, phagesensitive P. pestis, still virulent for guinea pigs, was recovered from one of two animals sacrificed at 40 days. The gross and histological lesions are described.