During a special two-day hunt (11, 12 November 1989) in Saint Croix State Park, Minnesota (USA), one side of the neck for each of 146 white-tailed deer (Odocoileus virginianus) was examined for ticks. Of the 5,442 ticks collected, 90% (4,893) were the winter tick, Dermacentor albipictus, and 10% (549) were the deer tick, Ixodes dammini, the primary vector of the causative agent of Lyme disease, Borrelia burgdorferi. Adult males had the greatest frequency of infestation of either D. albipictus (100%) or I. dammini (88%) and had on average more ticks, compared to other deer. Based on an examination of midgut material from 435 I. dammini by polyclonal antibody analysis, spirochetes were observed in 22% of the ticks. Species-specific monoclonal antibody analysis of the spirochetes confirmed that the bacteria were B. burgdorferi.