The prevalence of Theileria spp. infection was studied in sheep in the South Khorasan province in Iran from 2003 to 2004. A total of 840 sheep from 34 flocks were clinically examined and investigated for the presence of Theileria spp. in the appropriate blood smears and any tick species on the body of the animals. In this study, 11.9% of sheep were infected with Theileria spp., with a parasitemia of 0.02-0.1%. Differences in the infection rates were statistically significant among different areas of the South Khorasan province. The highest prevalence was found in the Ferdows area (31.4%) and the lowest rate in the Nehbandan area (0.7%). The prevalence of Theileria spp. infection in males and females and between different age groups of sheep were not statistically significant. Seasonally, the prevalence of Theileria spp. infection in sheep reached its highest level in June (26.3%), whereas it decreased in July and August. It was found that 50.5% of the animals harbored Rhipicephalus sanguineus, 48.5% harboured Hyalomma anatolicum and 0.89% harboured Hyalomma dromedari.