Theileria parva, the main protozoan responsible for East Coast fever (ECF) of cattle, is the most important livestock parasite in East and Central Africa. A high priority in ECF research is the development of methods for harvesting large numbers of T. parva sporozoites from the salivary glands of the tick vector, Rhipicephalus appendiculatus. Isolated sporozoites can be used for stabilate production (Cunningham et al., 1973, Int. J. Parasitol. 3: 583-587), for immunization of cattle using chemoprophylaxis (Cunningham, 1978. In Immunity to blood parasites of animals and man, L. H. Miller, J. A. Pino, and J. J. McKelvey (eds.). Plenum Press, New York, pp. 189-207) and for a variety of important biological and immunological studies. A difficulty in harvesting large numbers of sporozoites from ticks is that low infections of T. parva are usual in R. appendiculatus (Purnell et al., 1974, Int. J. Parasitol. 4: 513-517). During a study on the development of Theileria parasites in ticks, Young and Leitch (in press, Parasitology) found that temperature had a profound effect on the resultant infection levels in the salivary glands of the next instar. We present here findings which indicate that premoult and postmoult nymphal R. appendiculatus ticks exposed to natural conditions at Muguga or a certain diurnal temperature rhythm in the laboratory produce higher T. parva infections than those maintained under constant temperatures in the laboratory. Batches of engorged nymphal ticks infected with T. parva (Muguga), T. parva (Kiambu 4), or T. lawrencei (Serengeti transformed) described by Radley et al. (1979, Vet. Parasitol. 5: 117-128) were obtained by feeding nymphal R. appendiculatus from the laboratory strain at Muguga (Bailey, 1960, Bull. Epizoot, Dis. Afr. 8: 33-43) on cattle with high piroplasm parasitaemias (over 10% erythrocytes infected). Each batch used had completed repletion on one day. Groups of these batches of ticks were exposed, in open-ended, plastic tubes with nylon mesh to allow free air circulation, under grass cover of a paddock at Muguga. Other tick groups were incubated at constant temperatures of 23 C and 28 C + 0.5 C and approximately 85% relative humidity (RH) maintained by saturated potassium chloride solution (Winston and Bates, 1960, Ecology 41: 232-236). In addition, groups of ticks were incubated for alternate 12 hr periods by the use of three programmable, cooled incubators (Gallenkamp Ltd., London) at (1) 37-18 C, (2) 32-24 C, (3) 25-7 C, and approximately 85% RH. These diurnal temperature rhythms represent typical air temperature ranges occurring in (1) semi desert areas (Lake Magadi-ECF free), (2) coastal belt (Mombasa-marginal ECF area), and (3) Highland areas (Nanyuki or Muguga-ECF endemic or epidemic area) of Kenya. Gut smears were prepared from 10 nymphs from each group at daily or 2-day intervals depending on the rate of moulting (Young et al., 1980, Parasitology 81: 129-144), stained in Giemsa, and examined for developmental stages of Theileria. After the adult ticks had hardened, they were kept at approximately 20 C and 80% RH until they were examined for Theileria infections about 100 days after nymphal repletion. This was within the period of maximum infection levels for ticks infected with T. parva (Martin et al., 1964, Exp. Parasitol. 15: 527555). The adult ticks were applied to rabbits and removed 3 days after attachment. Their salivary glands were removed by dissection and stained with Feulgen's stain (Blewett and Branagan, 1973, Trop. Animal Hlth. Prod. 5: 27-34). The percent of infected ticks was determined in a group of 100 (50 males and 50 females) for each treatment and the total num-