An epidemiological investigation of fowl cholera was undertaken to compare its mode of spread with that of other infectious diseases. The work was divided into 3 parts: (1) Bacteriological and pathological tests to gain information on the characteristics of strains of Pasteurella obtained from typical fresh cases, the relation of these strains to the so-called "surface," "saprophytic" forms, the portal of entry, and the varieties of host response. (2) Field studies to ascertain the amount and character of natural infection, relation of P. avicida nasal carrier rate to spread, significance of different strains, and differences in response of individual birds to infection. (3) Experimental study of fowl cholera epidemics under controlled conditions to permit direct determination of the factors concerned in rise and fall of epidemic outbreaks. Studies so far carried out indicate that the epidemiology of fowl cholera rests on essentially the same basis as that of rabbit pasteurellosis. In each case the severe epidemic form is associated with a relatively virulent type of organism which survives with difficulty in the tissues of the host, whereas the endemic disease is associated with strains of relatively low virulence and high vegetative capacity. In fowl cholera, as in other animal diseases studied, the spread and severity of infection appear to be controlled by resistance of host and dosage of available organisms.