The rate of 58 neonatal calves in infection of Theileria sergenti was investigated in random samples on the farms located in Kyunggi, Chonbuk districts of Korea. 1. The criteria used in veryfying infection with T. sergenti included the detection of parasites by giemsa stain and acridine orange stain in the blood smear slides. 2. Further evidence of current or previous exposure to T. sergenti was based on demonstration of T. sergenti specific antibody and antigen by the western immunoblot and the directed immunofluorescent antibody test in the peripherial blood of the calves. 3. The prevalence rates were 35%, 50% in Kyunggi, Chonbuk provinces respectively and the overall prevalence in all the farms was 43.2％ by means of acridine orange stain. 4. The parasites that were observed in the peripherial blood of calves was showen surely by the western immunoblot to the characteristic 34KD antigen among the proteins of T. sergenti (Korean isolate). 5. And the antigen of the neonatal calves reacted at the very highest titer(1 : 2, 560) 6. These data highlight the significances of T. sergenti in the neonatal calf disease in Korea.