To analyze the resistance against Theileria sergenti infection in Holstein and Japanese Black cattle under field conditions, clinical, hematological and parasitological findings in these breeds of cattle that were grazed in the same pasture were examined during the grazing period. Holstein cattle showed a higher level of parasitemia than Japanese Black. With the progression of parasitemia, packed cell volume (PCV) decreased in both breeds and the minimum value of PCV recorded in Japanese Black was higher than in Holstein. The daily gain rate was higher for Japanese Black cattle than Holstein. These results suggest that the Japanese Black cattle display a higher resistance to T. sergenti infection than the Holstein under field conditions, based on a previous infection test carried out under experimental conditions without external factors.