A steer was infected with Theileria parva parva Kilae stabilate; nymphal Rhipicephalus appendiculatus ticks were applied to its ears so that they completed repletion when the steer had a high piroplasm parasitaemia. The engorged nymphs were subsequently incubated at 28°C for 26–29 days to complete moulting, when the adult ticks were divided into two groups; one was incubated at 18°C for 20 days and the other at 18°C for 14 days and then at 37°C for 6 days. Groups of ticks incubated at 37 and 18°C were triturated and each resultant supernatant fluid inoculated into a steer. Both steers became infected, but the 37°C supernatant group showed a much shorter pre-patent period to schizonts. Groups of ticks incubated at 37 or 18°C were applied to pairs of cattle for 24, 48 and 72 h and then removed. There was a more rapid transmission of theileriosis to cattle by ticks kept at high ambient temperatures compared to those kept at low ambient temperatures. All cattle on which ticks treated at 37°C were applied developed acute and fatal T. parva infection irrespective of the duration of tick application, while only 1 animal receiving ticks treated at 18°C and fed for 72 h developed infection. The pre-patent period for macroschizonts was very short in all the groups receiving ticks incubated at 37°C. Mature sporozoites were detected in the salivary glands of unfed ticks incubated at 37°C and after they were applied to cattle for 24, 48 and 72 h but in those incubated at 18°C sporozoites were detected, only at 48 and 72 h after application.