Babesiosis is a malaria-like illness caused by the intraerythrocytic parasite Babesia microti and is transmitted by the same tick that transmits Borrelia burgdorferi, the causative agent of Lyme disease. Babesiosis is well recognized in adult residents of southern New England and New York but has been described in only five children. To determine whether children are infected with B microti less often than are adults, a prospective serosurvey was carried out on Block Island, RI, where babesiosis is endemic. Randomly recruited subjects completed a questionnaire and provided a blood sample. Antibodies against B microti and B burgdorferi were measured using a standard indirect immunofluorescence assay and enzyme-linked immunosorbent assay, respectively. Of 574 subjects, 9% tested positive for B microti, including 12% of the 52 children (7 months through 16 years) and 8% of the 522 adults (not significant, P less than .6). Although babesiosis had not been diagnosed in any of the Babesia-seropositive subjects, 25% of the children and 20% of the adults reported symptoms compatible with this infection during the previous year. Of the 6 children and 45 adults seropositive for B burgdorferi, 17% and 14%, respectively, were also seropositive for B microti. It is concluded that children are infected with B microti no less frequently than are adults and that this infection is underdiagnosed in all age groups. Physicians who practice where Lyme disease is endemic should become familiar with the clinical presentation and diagnosis of babesiosis, both in adults and children.