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We determined the prevalence of Babesia microti and Borrelia burgdorferi in four populations of Ixodes dammini in eastern Massachusetts. The Feulgen's reaction was more sensitive than the Giemsa method for detecting salivarian Babesia. A combination of darkfield and direct-fluorescent-antibody examination proved more sensitive than either method alone for detecting spirochetal infection. The prevalence of spirochetes was greater than the prevalence of Babesia in each of the tick populations studied. Overall, 24% of nymphs and 47% of adults examined were infected with spirochetes; in contrast, 11% of nymphs and 14% of adults were infected with Babesia. The difference between the spirochetal and babesial prevalence was greatest in the most recently infested site. The rising incidence of Lyme disease, as compared to the stable incidence of human babesiosis, may result from the relatively greater abundance of Bo. burgdorferi infected I. dammini ticks in newly infested locations.