We determined whether the density of Lyme disease spirochetes varied between individual host-seeking deer ticks. Guts were dissected from 30 adult Ixodes dammini collected from three intensely zoonotic coastal Massachusetts sites, and the number of Borrelia burgdorferi present was estimated by a modified counting technique using indirect immunofluorescence. A median of 1,925 spirochetes was observed; ticks from the three sites contained similar numbers of spirochetes. No tick contained more than 4,500 spirochetes. Initial experimental reports establishing the efficiency of spirochetal transmission may have been based on ticks with a uniform spirochetal density, and extrapolations from these studies may thus overestimate the infectivity of host-seeking ticks in nature.