Trypanosoma theileri (Laveran, 1902) has been diagnosed in many countries and is commonly considered as a nonpathogenic hemoparasite, although some authors have described clinical signs in cattle infected with T. theileri. In April and May, 2005, 12 blood samples were received at the Exopol Diagnostic Laboratory (Zaragoza, Spain) from a Spanish bull-fighting farm located at Seville province. Clinical exploration of the animals revealed fever, progressive weight loss, anemia, and frequent recumbent position. Microscopic examination showed Theileria spp. in all cases (12), and in four of them, T. theileri was also observed. The clinical picture observed in the animals could be compatible with T. theileria infection. However, the contribution of T. theileri to the clinical signs seen at least in four cases is unknown. Further studies are necessary to determine the pathogenicity of T. theileri in the different animal species. To our knowledge, this is the first isolation of T. theileri in Spain.