Between 20 July and 15 Octoboer 1975, five cases of human infection with Babesia microti were diagnosed on Nantucket Island, Massachusetts. The illness was characterized by fever, drenching sweats, shaking chills, myalgia, arthralgia, extreme fatigue, and a mild-to-moderate hemolytic anemia. None of the patients had a history of splenetomy. Although all patients responded symptomatically to treatment with oral chloroquine phosphate, parasitemia and fatigue frequently persisted for several weeks to months.