At a major endemic focus in New Jersey, 50% of 290 adult Ixodes dammini collected in the fall of 1984 were infected with the Lyme disease spirochete (Borrelia burgdorferi), which was statistically higher than the rate found in the 202 adult ticks (39.6%) examined during the spring. Neither sex nor site of collection within the focus significantly affected the infection rate. The observed infection rates were similar to those reported in endemic areas of New York and Connecticut. Borrelia burgdorferi also infected all active stages of Amblyomma americanum ticks. Rates of infection were 5.4% in adults (n = 467) and 3.4% in nymphs (n = 289); 15.6% of clusters of unengorged larvae harbored B. burgdorferi, suggesting transovarial passage of the spirochete. Comparison of the rates of infection in I. dammini and A. americanum and their potential impact on Lyme disease transmission is discussed.