1. Certain bacilli within one strain may lose their flagella and become non-motile. 2. The change in serological behavior associated with the change in motility indicates that the flagella possess an individual antigenic property. 3. Flagella can be separated from the bodies of the bacilli. 4. A strictly flagellar antiserum can be produced by the injection of a flagellar suspension. 5. This direct production of a strictly flagellar antiserum is proof of the existence in flagella of a separate specific antigen.