In order to derive direct evidence implicating Ixodes dammini as a vector of human babesiosis, we determined the prevalence of Babesia microti infection in nymphal I. dammini collected on Nantucket Island. In experiments in the laboratory we found that nymphs remained attached to hamsters for about 3 days. Babesial infection was transmitted more often during 54 hours of attachment then during 36 or 48 hours. Since parasites were demonstrable in salivary glands solely after 48 hours, we derived an engorgement index for identifying ticks attached for 2 days or more. Of 156 nymphal I. dammini, collected from white-footed mice in 1979, 86 were engorged sufficiently to satisfy this index of attachment, and the salivary glands of four contained B. microti parasites. This demonstrates that about 5% of nymphal I. dammini are infected in nature. Risk of human infection can be reduced by prompt removal of attached ticks.