The prevalence of Haemophilus somnus in the genital tract of slaughtered and live cows in southern Ontario was investigated. The vagina and uterus of slaughtered cows were swabbed separately. Live cows were examined and sampled in two field surveys: Centre A and Centre B. In the former, aspirated mucus secretions and in the latter, specimens obtained by guarded swabbing were examined bacteriologically. Haemophilus somnus was isolated from 28 genital tracts of 461 slaughtered (6.1%), and seven of 199 live (3.5%) cows during the centre B survey. The isolates were recovered from both normal and diseased reproductive tracts. Fourteen strains isolated from genital organs were examined for pathogenicity in vivo to test the occurrence of pathogenic isolates. In the initial stage of the in vivo study on pathogenicity, each of the fourteen isolates was examined on one calf using an intracisternal inoculation. Subsequently, one pathogenic and one nonpathogenic strain were inoculated into five calves each to statistically confirm their pathogenic potential. Of 14 genital isolates of H. somnus examined in an intracisternal calf assay, six (43%) caused a fatal peracute neurological disease, while eight were nonpathogenic. A comparative pathological study of pathogenic and nonpathogenic isolates showed that the former caused a severe fatal suppurative meningoencephalitis whereas the latter caused no lesions whatsoever or a mild leukocytic leptomeningitis. The salient data obtained in this study indicate that there are pathogenic strains of H. somnus in the genital tract of apparently normal cows as well as of those with inflammatory disease.