Five groups of adult Rhipicephalus appendiculatus Neumann, with an average of 98 Theileria parva-infected salivary gland acini per tick, were exposed to four different temperature conditions in the laboratory or to quasi-natural conditions in Kenya. The survival of the infected ticks and T.parva parasites in their salivary glands were determined over time. Ticks kept underquasi-nutural conditions survived up to 86 weeks. This period was longer than that of ticks exposed to any of the four laboratory conditions, except the diurnal temperature rhythm of 13-23 degrees Celsius and 85% relative humidity (RH) to which it was comparable. Theileria parva survived for 82 weeks in ticks exposed to quasi-natural conditions, a period which was much longer than that of parasites in ticks maintained under any of the laboratory conditions. survival of the infected ticks and their infections seemed to be adversely affected by higher temperatures in the laboratory or drought in the field. There was an apparent density-dependent influence in parasite survival, with a dramatic fall in infection occurring in the more highly infected ticks before stabilizing at lower levels.