Anaplasmosis is the most widely distributed of several important tick-borne diseases that constrain cattle production throughout much of the world. Evaluation of the effectiveness of disease control strategies that integrate vaccination with tick control requires the ability to monitor tick and cattle infection rates. To detect Anaplasma marginale in ticks and bovine erythrocytes, a 2-kilobase DNA fragment from a cloned A. marginale gene coding for a surface protein having a Mr of 105,000 was prepared and evaluated as a probe. The probe was species specific and detected A. marginale DNA derived from infected bovine erythrocytes and adult Dermacentor ticks infected either as nymphs or adults. Tick infection was confirmed by microscopy and test feeding on a susceptible calf. The sensitivity of the probe is suitable for detecting infected ticks in experimental and field epizootiology studies.