This study was made to determine the population of ticks in infected sheep and attempting to identify the tick vectors of ovine theileriosis in an endemic area of Iran from 2000 to 2001. A total of 188 suspected cases of ovine theileriosis from 28 flocks were clinically examined and investigated for the presence of Theileria lestoquardi in appropriate blood smears and any tick species on body of sheep. In this study, 36.17% of sheep were infected to T. lestoquardi with a parasitemia of 0.01-15%. There was no significant differences between the rate of parasitemia in sheep and the frequency of infected ticks. We found that 61.1% of the animals harboured Hyalomma a. anatolicum, 33.42% Rhipicephalus sanguineus and 0.05% Hyalomma m. marginatum. The examination of 345 tick salivary glands showed that (15%) of salivary glands of H.a. anatolicum and (4%) of R. sanguineus contained Feulgen positive bodies. Seasonally, the prevalence of Theileria infection and H.a. anatolicum infestation in sheep reached the highest level in July (62.5%) and June (23.6%), while a decrease was observed in September (24.5%) and August (17.39%), respectively. The prevalence of Theileria infection in all age groups and between male and female sheep was statistically non-significant.

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