The hematophagous blackfly Simulium buissoni causes skin lesions on an island in the Marquesas archipelago that is holoendemic for hepatitis B virus (HBV). To test the hypothesis of the possible role of this fly in the transmission of hepatitis B, 506 children (age range 2–11 years) were examined for the presence of skin lesions, and attempts were made to detect HBV DNA in and on blackflies using two polymerase chain reaction methods. The mean number of skin lesions showed a positive correlation with the age of these children (r = 0.12, P < 0.05). Furthermore, it was significantly higher in the rural zone than in the urban zone (mean ± SD 41.02 ± 31.71 versus 17.73 ± 13.43; P < 0.05), and showed a correlation with a higher infection rate (73.9% versus 41.3%). Of the 45 pools of 10 insects tested, HBV DNA was not detectable on the inside of the insect, but was detectable on the flies (1–10 particles/insect in three positive pools). Infection by HBV conveyed by the flies is theoretically possible, but their indirect role via the numerous skin lesions caused on children is likely to explain such a high level of transmission.