1. A bacteriological study has been made of 210 fresh strains of Pasteurella obtained from typical cases of fowl cholera on seven widely separated poultry farms.
2. The strains have proved identical in consisting of small, pleomorphic, bipolar staining, Gram-negative, non-motile bacilli. They grew rapidly in infusion broth plus a trace of hemolgobin. They formed acid but no gas in media containing dextrose, saccharose, and mannite; indol was produced.
3. The strains fall into three distinct groups, according to their colony formation on hemoglobin agar. The "fluorescent" colony was large, whitish, opaque, exhibiting, under suitable conditions marked fluorescence. The "blue" colony was smaller, clear slate-blue, and non-fluorescent. The "intermediate" colony was moderately fluorescent at 15–18 hours growth, and "blue" thereafter. It was "blue" at all times when crowded and occasionally of "ring" form. "Fluorescent" colony cultures developed "blue" colony forms under certain conditions; otherwise all forms were stable.
4. Strains from "fluorescent" colonies were resistant to precipitation by acids, to sedimentation by centrifuging, and although they combined with specific antiserum, did not agglutinate. They were relatively highly virulent and occurred in flocks where fowl cholera was epidemic.
5. Strains from "blue" colonies were precipitated by acids over a wide range of concentration and agglutinated strongly in antisera. They were relatively of low virulence and occurred in flocks where fowl cholera was endemic.
6. Strains from "intermediate" colonies varied in behavior betweee the "fluorescent" and "blue" strains. They came from a flock where fowl cholera was epidemic.