A comparison was made between the development of Theileria mutans (Aitong) in adult Amblyomma variegatum ticks and the development of T. parva (Muguga) in adult Rhipicephalus appendiculatus ticks. Both batches of ticks dropped as engorged nymphs from cattle whose intraerythrocytic parasitemias were approximately 40%, and it was thus possible to make a direct comparison of the infection rates produced in their tick hosts in this experiment by the 2 species of Theileria. Ticks of the 2 batches were fed in ear bags on rabbits, and equal numbers of male and female ticks were removed daily for 5 days. The salivary glands were dissected out from these ticks, and from unfed ones in each batch, processed, and examined for Theileria parasites, which were categorized according to their morphological maturity. The A. variegatum ticks had a lower rate of infection with Theileria. Fewer parasites were observed in the ticks, and they took longer to mature. When the development of the 2 species of Theileria was compared, the major difference appeared to be that T. mutans had large Babesia-like infective particles.