A clinical picture strongly suggestive of typhoid in a patient hospitalized because of suspected typhus led to bacteriologic and serologic studies that established the illness as paratyphoid A. This patient had become ill within four days of his arrival on Guam from Okinawa. When further inquiry revealed that other marines and soldiers, recently evacuated from Okinawa, were in four hospitals on Guam because of similar clinical findings, the obvious importance of learning whether these patients had typhoid or paratyphoid led us to undertake an investigation of this problem. Bacteriologic and serologic studies established typhoid or paratyphoid A as the diagnosis for 24 cases. Of these, 21 were paratyphoid A and 3 were typhoid. It is our purpose in this paper to present certain clinical, bacteriologic, immunologic and epidemiologic data that relate to these 24 proved cases of typhoid or paratyphoid A in immunized military personnel.1 DATA RELATING TO THE