The development at 28 degrees C of Theileria annulata (Hissar) in the salivary glands of its tick vector, Hyalomma anatolicum anatolicum, was studied using Giemsa-stained smears, methyl green-pyronin-stained preparations of whole glands, and electron microscopy. Nymphs which had engorged on T. annulata-infected calves showed kinetes in the haemolymph from Day 7 to Day 14 post engorgement, when all ticks had completed moult. Intracellular sporonts were observed within salivary gland acini from Day 7 onwards and these developed by rapid nuclear division and cytoplasmic proliferation to form primary sporoblasts. Further development was stimulated by feeding on a rabbit or by incubation at 36 degrees C. The primary sporoblasts appeared to become organized into membrane-bound subunits. Within 48 hours of attachment to the host, or 72 hours of 36 degrees C incubation, these units dissociated to form secondary sporoblasts. The final phase of development resulted in the progressive formation of discrete, uninuclear sporozoites within these secondary sporoblasts. No morphological differences were observed in parasite maturation between the fed and incubated groups although development was retarded by at least 24 hours in the latter. In the incubated group there was also a marked decrease in the degree of synchrony of development which resulted in fewer sporozoites being present at any one time.