The incidence of infection in adult dairy cattle in New York State with Trypanosoma (Megatrypanum) theileri (Laveran 1902) was determined by culturing buffy coats of peripheral blood samples in tissue culture growth media. Three sample groups of cattle were studied and revealed an overall rate of trypanosome infection of 44.4% (67 of 151) as determined by a single culture. Fifty-seven cows, representing four herds from three different geographic locations in the Southern-tier region of the state, comprised the first group. The infection rates of these herds ranged from 56.5 to 100%. The second group consisted of 81 cows randomly selected from animals admitted to the Large Animal Clinic of the New York State College of Veterinary Medicine. Trypanosoma theileri was cultured from 25.9% (21 of 81) of these animals. Repeated blood cultures from a third group of thirteen yearling heifers during the spring and summer revealed an increase in the infection rate from 0% in May, to 66.7% (8 of 12) in September, with 76.9% (10 of 13) positive at least once during this period. These findings are compared with the reported incidence of bovine T. theileri infections in other areas of the United States and in other countries.