In 1906 we discovered the existence of actinomycosis among the black mountain sheep of northern British Columbia. Two specimens out of six were badly affected, the bones of the jaws being greatly enlarged, and perforated by deep pits. The black sheep of the Stickine and Iskoot regions are so seldom seen by white men, save when a sportsman kills his allotment of three specimens, we really do not know anything about the extent to which actinomycosis prevails in those herds, or how deadly are its effects. One thing seems quite certain, from the appearance of the diseased skulls found by the writer in the taxidermic laboratory of Frederick Sauter, in New York. The enormous swelling of the diseased jaw bones clearly indicates a disease that in some cases affects its victim throughout many months. Such a condition as we found in those sheep could not have been reached in a few days after the disease became apparent. Now, in our antelopes, the collapse and death of the victim usually occurred in about 10 days from the time that the first swelling was observed: which means a very virulent disease, and rapid progress at the climax. The jaw of one of our antelopes, which was figured in Dr. Blair's paper in the Eleventh Annual Report of the New York Zoological Society (1906) shows only a very slight lesion, in comparison with those of the mountain sheep. The conclusion is that among the sheep, this disease does not carry off its victims in any short period like 10 days. The animal must survive for some months after it becomes apparent. At least two parties of American sportsmen have shot rams afflicted with this disease, but I have no reports of any sheep having been found dead from this cause. This disease is well known among domestic cattle, but so far as we are aware it never before has been found among wild animals. The black sheep herds wherein it was found in British Columbia are absolutely isolated from domestic cattle and all their influences, and therefore it seems quite certain that the disease developed among the sheep spontaneously,--a remarkable episode, to say the least. Whether it will exterminate the black mountain sheep species, and in time spread to the white sheep of the northwest, is of course a matter of conjecture; but there is nothing in the world to prevent a calamity of that kind. The white sheep of Yukon Territory range southward until in the Sheslay Mountains they touch the sphere of influence of the black sheep, where the disease could easily be transmitted. It would be a good thing if there existed between the two species a sheepless zone about 200 miles wide.