1. A hemophilic bacillus has been regularly obtained in culture from the respiratory tract of a series of swine experimentally infected with swine influenza and from a small number of spontaneous field cases of the disease. It has not been observed in respiratory tract cultures from a group of swine free from influenza. 2. The cultural and morphological characters of the organism have been described and the name Hemophilus influenzas (variety suis) suggested. The organism exhibits marked serological diversity, since only two out of eight strains studied were serologically identical. It is usually non-pathogenic for rabbits and white rats, and irregularly pathogenic for white mice. One strain of the organism was pathogenic for guinea pigs while two others were not. 3. Eleven out of thirteen attempts to induce symptoms of disease in swine by intranasal inoculation with pure cultures of H. influenzae suis were entirely negative. The remaining two attempts which suggested a positive result have been discussed. 4. Attention has been called to the marked similarity which exists between non-indol-producing strains of H. influenzae and H. influenzae suis.