Equine piroplasmosis (EP), caused by two parasitic organisms, Theileria equi and Babesia caballi, is a tick-borne disease of recent concern in horses in the U.S.A. Outbreaks of EP have been detected in Florida, Missouri, Kansas and Texas. In 2009, EP transmission in Texas occurred through the adults of two tick species, Amblyomma mixtum [formerly known as Amblyomma cajennense (Fabricius, 1787)] Koch (Ixodida: Ixodidae) and Dermacentor variabilis (Say) (Ixodida: Ixodidae), the American dog tick (ADT). In this study, we developed a continent-scale map for the distribution of the EP vector species D. variabilis, using a presence-only modelling approach to assess the habitat preferences of this tick. We used identification records from our tick geodatabase of locations in which the presence of the ADT had been noted. The potential distribution of the ADT in the U.S.A. was estimated from environmental factors using the maximum entropy approach based on localities in which there is a high probability of occurrence according to habitat suitability. Elevation and temperature were found to be biologically significant environmental variables influencing the presence of this tick species. Properly designed and constructed probability surfaces using maximum entropy offer one useful approach to the mapping of distribution ranges of tick species based on suitable habitat in the U.S.A.