PART I. The relationship between tick infestation and the incidence of tick pyaemia in two tick -infested districts is discussed. The evidence obtained tends to confirm the opinion that tick -bite is a causal factor in the production of this disease. 1. The results of experiments with infected ticks showed that the tick is unlikely to act as a true vector of the causal staphylococcus in tick pyaemia. In ticks infected by feeding on an animal suffering from a staphylococcal septicaemia, the infection did not survive the moulting period. 2. Haemolytic staphylococci were found to be of common occurrence in the natural orifices of ewes and on the skins of lambs. 3. The staphylococci from the above sites were shown to have the same characteristics as the staphylococci from tick pyaemia cases. 4. A staphylococcal septicaemia was produced in one lamb by feeding ticks on skin contaminated with staphylococcal culture. The possible influence of tick -borne fever on the development of tick pyaemia is discussed. 5 . An examination of the manner in which the staphylococcal infection becomes generalised is made. An investigation of the properties of tick salivary gland secretion is recorded. 6 . A preliminary study of the pathology of tickbite is made. PART III. Staphylococcal toxoid administered either to the lamb or to the ewe before lambing was shown to be of no value in the prevention of tick pyaema on Scottish hill sheep farms.