Populations of host-seeking blacklegged tick, Ixodes scapularis (Say) and lone star tick, Amblyomma americanum (L.), nymphs were monitored at selected sites for 4 yr and at additional randomly selected sites in the Greenbelt National Park, MD for two of those years. Ticks collected from the random sites during the second year of the study were tested for the presence of human pathogens. Borrelia burgdorferi Johnson et al. was detected in 22.2% of the I. scapularis nymphs collected while Anaplasma phagocytophilum Foggie (Dumler et al.) was detected in 3.7%, and one nymph was coinfected with both pathogens. No I. scapularis nymphs tested positive for Babesia microti (França) and no A. americanum nymphs tested positive for Ehrlichia spp. In the years when both random and nonrandom sites were sampled (sampled ≤2 d apart, n= 14 d), significantly more A. americanum nymphs (P= 0.003) were captured at the nonrandom sites than at the random sites; no difference (P = 0.2415) was found for I. scapularis nymphs. No density effect due to vegetational communities was found for nymphs of either species of tick. Host-seeking nymphs of both species of ticks were abundant the first year of flag sampling, dropped dramatically in numbers the second year, and gradually increased (particularly A. americanum) the following 2 yr. The annual variations in tick densities demonstrate the value of early season–monitoring of tick populations on park premises, which affords park managers an opportunity to take appropriate measures in the event of a year of high tick abundance.