Abstract: The objective of this study was to estimate the prevalence of antibodies against Anaplasma marginale in sera from white-tailed deer (Odocoileus virginianus texanus) in the states of Nuevo Leon and Coahuila, Mexico. One hundred and sixty five deer from 5 farms were live-trapped in 2005 using the net-canyon method. Blood was extracted by jugular venipuncture using vacuum tubes. Antibodies in the sera were detected by the indirect immunofluorecent technique. Seroprevalence was high because 69.7% of deer reacted positively to A. marginale. The prevalences for the municipalities of Anahuac and Acuña were 61.9 and 83.3%, respectively. The risk of a seropositive animal was 3 times greater in the municipality of Acuña. The prevalence between sexes were equal (p>0.05). In conclusion, a high frequency of antibodies against A. marginale in white-tailed deer in northeastern Mexico was found. More studies are recommended with the aim of determine the risk of deer in the transmission of A. marginale to cattle.