An investigation has been made into the distribution and bionomics of Ornithodoros moubata (Murr.) in East Africa in relation to the incidence of relapsing fever, and a survey has been made of the infestation in over 4,600 African huts, together with the temperature and relative humidity conditions.In Kenya Colony, relapsing fever is endemic in the high rainfall areas of Meru, Nyeri and Taita Districts. These habitats are cool and wet with a mean microhabitat temperature of 71°F. and a relative humidity of 86 per cent. Tick infestations were relatively sparse and were rare in the hot and dry climate of Embu District, the base of the Taita Hills and generally over all such country in Kenya.In Tanganyika Territory, relapsing fever is widespread, and the most striking difference was the relatively much greater abundance of the tick, especially in the dry central areas. It is pointed out that although relapsing fever is most prevalent in the north-west, endemicity is at a lower level than in Kenya, and decreases towards the south-east, indicating that the degree of incidence of the disease does not conform with that of the vector.In the Digo District, south of Mombasa on the Kenya coast, ticks showed a reversal in their choice of microclimate from those in the cool highlands and were numerous in hot, moist conditions. The incidence of the disease was very low.O. moubata was widespread in the Usambara Mountain area of Tanganyika. Ticks were most numerous in the cool, wet conditions above 4,000 ft., but were also abundant in the hot, moist foothills and plains, whereas they were absent in hot and dry country at the base of the Taita Hills in Kenya 80 miles to the north.As humidity appeared to be a foremost factor affecting the distribution of O. moubata it was not possible to evaluate clearly the effects of temperature. It is suggested that all the conflicting evidence of the relationship of the tick populations to microclimate and the incidence of relapsing fever may be explained only by introducing a hypothesis of biological variation in the tick itself. It is shown that there are two peaks of greatest abundance, at relative humidities of 86 and 67 to 68 per cent. respectively, and it is suggested that these two peaks represent the distribution of two hypothetical hut-haunting biological forms.An examination of the blood-meals from pooled catches by the precipitin test showed that in the cool and wet habitats of the Kenya highlands and the north-west of Tanganyika, 94 per cent. of the recognisable feeds were on man and only 2 per cent. on fowls. In the hot and moist habitats of Digo and the low-lying area between Digo and the Usambara Mountains, 18 per cent. were on man and 78 per cent. on fowl. In the mainly warm and moist habitats of the Usambara Mountains and the area bordering the south-east of Lake Victoria, 73 per cent. were on man and 22 per cent. on fowls.It is therefore suggested that there are two biological forms of O. moubata found in huts, one feeding on man and the other feeding on fowls. The former is found in huts at high altitudes in areas having a cool and wet climate; it is essentially a human parasite showing a marked preference for the blood of man while ignoring the presence of fowls however numerous or available. It occurs in greatest abundance at a relative humidity of about 86 per cent. It is found at relatively low temperatures from 67° to 75°F. It is absent in areas where the microclimate is consistently over 90 per cent. R.H. and may not occur where it is consistently lower than about 74 per cent.The form that feeds on fowls appears to possess a tolerance to a wide range of temperature and R.H., occurring in greatest abundance at 67 to 68 per cent. R.H. It is found at temperatures from 68° to 87°F. It is more resistant to starvation than the form that feeds on man.