A total of 466 cows and 286 buffaloes, suffering from endome­tritis, metritis, retained placenta, and pyometra were investigated to iso­late and identify the causative agent. Moreover, the pathogenicity of the isolate was studied in guinea pigs and rats which were intraperitoneally (IP) or intracerebrally (IC) injected with 0.5 ml of 24 hours bactothiol broth culture. Specimens from the uterus, brain, kidneys, lungs, and spleen of the inoculated laboratory animals were collected and fixed in 10% neutral buffered formol saline for histopathological examination. Haemophilus somnus was isolated from cows and buffaloes showing re­tained placenta and pyometra and from cows suffering endometritis and abortion with overall percentages of 4.93 for cows and 1.75 for buffa­loes. The encountered lesions in guinea pigs were hemorrhages and congestion, besides degeneration of the internal organs. In rats, the le­sions were nearly similar to those in guinea pigs besides oedematous and congested uterine lamina propria, demyelination of the brain, and hemor­rhagic and emphysematous lungs.