An epizootic of anaplasmosis affecting 18 of 90 beef cows in winter on a western Illinois ranch was investigated to determine the probable source and mode of transmission. The cause of the epizootic was Anaplasma caudatum. Cows were classified as sick, convalescent, or carrier on the basis of blood smear, complement fixation, or modified rapid card agglutination test results. Patterns of movement did not suggest a common exposure prior to assembly of cattle at the affected ranch. The combination of clinical stages and temporal pattern of the epizootic was compatible with exposure on one or more occasions after arrival of cattle on the ranch, most likely during herd vaccination or ear tagging. A serologic testing and treatment program was initiated to rid the herd of infection. Seventeen surviving cows were treated 3 times with 20 mg of oxytetracycline/kg, IM, at 5-day intervals. At the end of the 6-month follow-up period, 1 of 17 cows was still serologically positive and was treated with a second regimen of oxytetracycline. We believe that eradication of A caudatum was successful, because no clinical cases of anaplasmosis have been reported on the ranch during the last 3 vector seasons.