T. theileri has been reported as a parasite of cattle from many parts of the world. The parasites generally are scanty in the peripheral blood and more readily detected by cultural methods than by microscopical examination of stained smears (Herbert, 1964; Wells, 1971). The presence and distribution of the parasite in Iran has not been reported previously. This communication presents the results of studies which have been undertaken to find T. theileri and to determine its seasonal distribution in this country.