In spite of the detailed study which it has received, the life cycle of _Babesia bovis_ has not been satisfactorily worked out. The asexual reproduction in the blood of the vertebrate host has been described but the cycle in the tick is practically unknown. More successful attempts have been made to work out the life cycle of a related species, _Babesia canis_, which causes malignant jaundice in dogs in Africa and parts of Southern Europe. In this instance, also, the disease is transmitted by heredity to the ticks of the second generation. Yet the larval, or "seed ticks," from an infected female are not capable of conveying the disease, but only the nymphs and adults. Still more complicated is the condition in the case of _Babesia ovis_ of sheep, which Motas has shown can be conveyed solely by the adult, sexually mature ticks of the second generation. In _Babesia canis_, Christopher (1907) observed developmental stages in the tick. He found in the stomach of adult ticks, large motile club-shaped bodies which he considered as oökinetes. These bodies pass to the ovaries of the tick and enter the eggs where they become globular in form and probably represent an oocyst. This breaks up into a number of sporoblasts which enter the tissues of the developing tick and give rise to numerous sporozoites, which collect in the salivary glands and thence are transferred to the vertebrate host. A number of other species of _Babesia_ are known to infest vertebrates and in all the cases where the method has been worked out it has been found that the conveyal was by ticks. We shall not consider the cases more fully here, as we are concerned especially with the method of transfer of human diseases. TICKS AND ROCKY MOUNTAIN SPOTTED FEVER OF MAN--Ever since 1873 there has been known in Montana and Idaho a peculiar febrile disease of man, which has gained the name of "Rocky Mountain spotted fever." Its onset is marked by chills and fever which rapidly become acute. In about four to seven days there appears a characteristic eruption on the wrists, ankles or back, which quickly covers the body. McClintic (1912) states that the disease has now been reported from practically all of the Rocky Mountain States, including Arizona, California, Colorado, Idaho, Montana, Nevada, Oregon, Utah, Washington, and Wyoming. "Although the disease is far more prevalent in Montana and Idaho than in any of the other States, its spread has assumed such proportions in the last decade as to call for the gravest consideration on the part of both the state and national health authorities. In fact, the disease has so spread from state to state that it has undoubtedly become a very serious interstate problem demanding the institution of measures for its control and suppression."