A 48-year-old woman was bitten by a tick on Nantucket Island, Massachusetts, and developed a fever. Babesia microti were recognized in her erythrocytes. Transmission to rodents confirmed the diagnosis of babesiosis. This is the sixth reported example of this malaria-like, tick-transmitted disease and the second to occur on Nantucket. The patient had not received transfusions or traveled to an endemic malaria area. Hemolytic anemia, present in this case, was the cause of death in three previously reported cases. Chloroquine therapy was successful in eradicating the parasites in this patient, the third survivor.