Mohler has shown that the disease under the names of “slow fever” and “mountain fever” has existed in Texas and New Mexico for at least twenty-five years. ETIOLOGY AND EPIDEMIOLOGY =Etiology.=—The causative organism, _Micrococcus melitensis_, is a small coccus, rather oval than round and about 0.4 micron in diameter. In morphology it is quite variable and may occur in pairs or in short chains and is Gram-negative. It emulsifies evenly and rapidly in a hanging-drop preparation and is nonmotile. Possibly on account of its showing a rather active Brownian motion there has been a reporting of slight motility by some authorities. Very striking is the characteristic of very slow growth so that cultures on agar fail to show colonies before the fourth day. These minute transparent colonies become somewhat opaque and about 1/10 inch in diameter by the tenth day. Gelatine is not liquefied and litmus milk is not altered. The optimum reaction of media is about +0.75 to phenolphthalein and it grows best at the body temperature. It has great powers of resistance to drying so that it survives in dust for long periods. Horses, cows, asses, as well as goats, are susceptible. It is very difficult to infect rabbits, mice and guinea pigs. Monkeys have been chiefly utilized in experimental work. It would appear as if there were other organisms closely related to _M. melitensis_ and a great deal is now being written as to confusing serum reactions from the use of _M. paramelitensis_. Evans and others have studied the relationship between _Bacillus abortus_ and _M. melitensis_. Morphologically and culturally these organisms are quite similar and Evans has demonstrated a marked degree of cross-agglutination. This is a probable explanation of the finding by Kennedy of agglutinating power in the sera and milk of certain cows, but inability to isolate _M. melitensis_ from the agglutinating milk. =Epidemiology.=—Many experiments have failed to show any mosquito, biting fly or louse as a probable factor in the transmission of the disease. The infection is readily transmitted by subcutaneous inoculation so that in a case in goat or man, with the cocci in the peripheral circulation, it is reasonable to suppose that a biting insect might transfer the infection by going directly from one animal to another. There have been several laboratory infections, but when we consider that of the great number of cases treated at Haslar hospital and elsewhere in England, with frequent elimination of the organism in the urine, and practically no infections among the friends or attendants, it would seem as if usual methods of infection were inoperative. There does not seem to be a carrier problem in this disease. Urine showing bacterial contamination, when dried and mixed with dust, has caused infection and contaminated urine applied to the glans penis of a monkey caused the disease.