Theileria sergenti causes persistent theileriosis in cattle, characterized by fever and chronic anemia. Theileriosis causes losses in feed efficiency and growth retardation through cycling infections in endemic areas. Among several major proteins of T. sergenti merozoites, the surface protein p33 is reported to be the most immunogenic. In this study, we investigated the use of p33 as a diagnostic antigen in a latex agglutination test to monitor antibodies against T. sergenti. When compared with TaqMan polymerase chain reaction, the sensitivity and specificity of the latex agglutination test were 86.5 and 92.5%, respectively. An epidemiological survey using the latex agglutination test was conducted with 1,046 sera collected from 4 slaughterhouses and 2 individual pasture farms throughout South Korea; 27.3% of samples were seropositive, depending on the areas in which the cattle were raised. This study indicated that the latex agglutination test could be used as a convenient tool for epidemiological monitoring of T. sergenti infections in the field.